What Happens to Stock When Companies Merge: Shares, RSUs and Taxes

By Kevin Luu, Co-Founder and Chief Learning Officer. Reviewed on 2026-09-30.

In a merger, your shares of the company being bought are converted into whatever the merger agreement says: cash at a fixed price per share, shares of the buyer at a fixed exchange ratio, or a mix of both. Your old shares stop trading on the closing date, and the new cash or shares typically land in your brokerage account a few days later. The tax result depends on the form of payment: an all-cash deal is a taxable sale, while an all-stock deal that qualifies as a tax-free reorganization generally lets you defer the gain until you sell the new shares.

For a California or Washington family earning $750,000 or more, the question is rarely about a single brokerage position. It is usually about employer stock: vested shares from RSUs and ESPP, unvested RSUs, and options, all at a company that has just announced it is being acquired. Biotech and tech acquisitions are common enough that many families in the Bay Area and Seattle will go through at least one. This guide explains what happens to each piece, with a worked example.

Three ways a merger can pay shareholders

The rules for stock deals come from the reorganization sections of the tax code. Section 354 provides that no gain or loss is recognized when stock is exchanged solely for stock in a qualifying reorganization, Section 358 carries your old basis over to the new shares, and Section 356 taxes any cash (“boot”) received, with gain recognized only up to the amount of cash and no loss allowed (IRC §356). Whether a deal qualifies is decided by the companies and their lawyers under IRC §368, not by you. The proxy statement or prospectus will say which treatment the companies expect.

After closing, the issuer usually posts IRS Form 8937 on its investor relations site. It explains how the deal affects your basis, and your tax preparer will want it.

What happens to the share price before closing

On announcement, the target’s stock typically jumps toward the deal price but trades slightly below it until closing. That gap reflects the time until closing and the chance the deal fails on regulatory review or a shareholder vote. In a stock deal, the target trades roughly in step with the buyer’s shares multiplied by the exchange ratio, so your eventual value moves with the buyer’s stock price until the day of closing.

The buyer’s stock can move either way. If the market thinks the buyer overpaid or took on too much debt, it may fall. That matters more in a stock deal, because you are about to become a buyer shareholder.

What happens to RSUs, options and ESPP in an acquisition

Employee equity follows the merger agreement and your company’s equity plan, not the general shareholder rules. The common outcomes are:

• Vested shares you already own (from RSU vests or ESPP purchases) are treated like any other shareholder’s stock.
• Unvested RSUs are usually assumed or converted into buyer RSUs at the exchange ratio, keeping the same vesting schedule. Less often they are cashed out at the deal price, and some plans accelerate vesting. “Double-trigger” acceleration typically vests them only if you are let go without cause (in some plans, or resign for good reason) within a set window after closing. Check your grant agreement.
• RSUs that are cashed out or accelerated are wages when paid. Federal withholding is typically 22% on supplemental wages, or 37% on the part above $1,000,000 in the calendar year, for 2026 (IRS Pub 15). At a 35% or 37% bracket, 22% withholding usually leaves a balance due in April.
• Stock options are commonly converted into options on buyer stock. For incentive stock options, a substitution that follows IRC §424(a) (the spread is not increased and no extra benefits are added) keeps them ISOs. Underwater options may be cancelled.
• ESPP plans often end the current offering period with a final purchase shortly before closing.

One trap: if ISO or ESPP shares are cashed out before you meet the holding periods (two years from grant and one year from exercise or purchase, under IRC §422(a)(1) and §423(a)(1)), that is a disqualifying disposition. Part of the gain is then taxed as ordinary income rather than long-term capital gain. You do not choose the closing date, so the timing is out of your hands.

Worked example: a cash deal for a $1 million household in California

Assumptions (illustrative): a married couple filing jointly in San Mateo County. One spouse works at a mid-size biotech that agrees to be bought for $100 per share in cash, closing in 2026. Before the deal, federal taxable income is $850,000 and California taxable income is $900,000. They own 4,000 vested shares held more than one year with a basis of $40 per share, and 1,500 unvested RSUs that the agreement cashes out at $100.

1. Vested shares. Proceeds $400,000, basis $160,000, long-term gain $240,000.
2. Federal tax on the gain. Their taxable income is already above $613,700, so the whole gain is taxed at 20% for 2026: $48,000. The 3.8% Net Investment Income Tax also applies because income is well above the $250,000 joint threshold: $9,120 (IRS NIIT). Federal total: $57,120.
3. California tax on the gain. California has no lower rate for capital gains (FTB). Using 2025 schedules (the 2026 schedules had not been published as of this review), the gain falls in the 11.3% joint bracket: $27,120. Taxable income now crosses $1,000,000, so the 1% Behavioral Health Services Tax applies to the $140,000 above it: $1,400. California total: about $28,520.
4. Cashed-out RSUs. $150,000 of wages. Federal withholding at 22% is $33,000; at a 37% marginal rate the federal tax on it is $55,500, so about $22,500 more is due at filing. California SDI takes 1.3% ($1,950) with no wage cap in 2026. The $150,000 is also California taxable income: stacked on top of the gain, California taxable income reaches $1,290,000, still in the 11.3% bracket and above $1,000,000, so it costs about 12.3% (11.3% plus the 1% Behavioral Health Services Tax), or $18,450. Medicare tax also applies: 1.45% plus the 0.9% Additional Medicare Tax, since their wages are already above the $250,000 joint threshold, or $3,525 for 2026 (IRS).

Combined, the cash deal creates roughly $85,640 of federal and California tax on the vested shares alone, in one year, with no choice about timing.

Same deal, all-stock instead. Suppose the buyer offered 1.25 of its shares per target share in a qualifying reorganization. The 4,000 shares become 5,000 buyer shares with a carryover basis of $160,000 ($32 per share). No tax is due at closing. The $240,000 gain is deferred until they sell, which lets them choose the year. The trade-off is that their family’s wealth is now tied to a single stock they did not pick.

Same deal, Washington family. The federal $57,120 is the same. Washington has no wage income tax in 2026, so the RSU cash-out owes no state income tax. Washington’s 7% capital gains tax applies only to long-term gains above the standard deduction, which was $278,000 for 2025; the 2026 amount had not been published as of this review (WA DOR). If the $240,000 is their only gain for the year, it sits below the 2025 deduction, so the state tax would likely be zero. Note that Washington has enacted a 9.9% income tax on AGI above $1,000,000 starting January 1, 2028 (WA DOR), so a deal closing in 2028 or later may look different.

The decisions a family actually has to make

• Sell now or wait for closing. Selling after announcement locks in a price slightly under the deal price and removes the risk that the deal fails. Waiting captures the full price. Either way the gain is taxable: in the year you sell, or in the year a cash deal closes.
• Cash or stock election. Some deals let shareholders elect cash or stock, subject to proration. In California, electing stock on a low-basis lot can defer a large bill; electing cash on a high-basis lot costs little in tax.
• Estimated payments. A cash closing plus cashed-out RSUs can leave the household well under-withheld. Many families make a federal and California estimated payment for the quarter the deal closes.
• Concentration after closing. If you receive buyer stock and keep working there, your salary, unvested equity and savings may all depend on one company. A written target for how much of your net worth can sit in one stock, and a schedule for getting there, makes that decision easier to follow through on.
• Where the cash goes. A large cash payout is a good moment to decide between investing at once and phasing in. Our guide on lump sum versus dollar-cost averaging walks through the trade-off.

Common mistakes

• Assuming a stock-for-stock deal is always tax-free. Only a qualifying reorganization is, and any cash, including fractional-share cash, can trigger taxable gain.
• Losing track of basis. Your broker’s reported basis for RSU shares can be wrong or missing, which can lead to paying tax on income that was already taxed as wages at vest. Keep your vest records.
• Trusting 22% withholding on a cash-out at a 37% bracket.
• Ignoring the buyer. In a stock deal, you are making an investment decision in the buyer, whether or not you meant to.

What to document for your kids

A merger is one of the few times a family’s biggest asset changes form overnight. Write a one-page Decision Memo: what the deal paid, what you elected, what you sold and kept, what it cost in tax, and why. Keep it with your basis records. In ten years, that page tells your children how you weighed certainty against upside, which a brokerage statement cannot. Conversations fade. Documentation compounds.

Frequently asked questions

Do I have to do anything when my company merges?

Often, no. In most cash or stock deals the conversion happens automatically at closing. You may be asked to vote, and in some deals to return an election form choosing cash or stock by a deadline. If you miss an election, you usually receive the default mix stated in the proxy. Read the shareholder materials and your equity plan notices as soon as the deal is announced.

What happens to my stock if the merger falls through?

Your shares stay shares of the original company and keep trading. The price usually falls back toward where it traded before the announcement, sometimes lower, because the market had priced in the deal. Employee equity also stays under the original plan. This risk is the main reason the target trades below the deal price before closing.

What is an all-stock merger?

An all-stock merger pays target shareholders only in shares of the buyer, at a fixed exchange ratio, with cash only for fractional shares. If it qualifies as a reorganization under IRC §368, shareholders generally owe no tax at closing, and their old basis carries over to the new shares. Tax is due later, when the new shares are sold.

Next step

If your employer has announced a deal, pull together three things this week: your grant agreements, your vest and purchase records with basis, and the proxy statement. Those answer most of the questions above. Bright Advisers helps families with equity compensation and benefits, tax planning and concentrated-stock risk; for the shareholder side of an acquisition, see what happens to stocks if a company is bought. If you want a second set of eyes, the Tax & Wealth Assessment covers deals like this one.

This is general education, not individual tax or investment advice. The right move depends on your family’s full picture, and on the terms of your specific deal and equity plan.

Written by

Kevin Luu

Co-Founder and Chief Learning Officer, Bright Advisers

Kevin has advised high-income W-2 tech and biotech families since 2015, and leads the education-first Age Five Family Office from Brea, California.

Connect on LinkedIn →  · About Kevin