What Happens to Your Stock in a Merger or Acquisition

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

When a company is acquired, your shares do not disappear. At closing they are converted into whatever the merger agreement says: cash, shares of the buyer, or a mix of both, at a fixed price or exchange ratio. A cash deal is usually a taxable sale on the closing date, while a stock-for-stock deal that qualifies as a reorganization is generally not taxed until you later sell the new shares.

For an employee, the shares you already own are only half of the question. Unvested RSUs, stock options and an open ESPP offering period each follow their own rules, set by the plan document and the merger agreement. This guide covers each piece, with a worked example for a California biotech family, the Washington contrast, and the decisions that are actually yours to make.

Why an acquisition matters more for a high-income employee family

Consider a household with $950,000 of W-2 income where one parent works at a Bay Area biotech and the other is a physician. A large share of their net worth may sit in one ticker: vested shares from past RSUs, unvested grants, and ESPP shares. An acquisition announcement changes the value, the tax timing and the vesting schedule of all of it at once, often with only a few weeks between the shareholder vote and closing.

Deals in this space are common. Large pharma companies buy smaller biotechs, and big tech companies buy startups and public software firms. If you work at a Genentech-size company, you are more often on the buying side; if you joined a smaller company for equity upside, you may be on the selling side.

What happens to shares you already own, by deal type

Whether a stock deal qualifies as a tax-free reorganization under IRC §368 is decided by the deal structure, not by you. The proxy statement or prospectus will say what the companies expect, and after closing the company typically posts IRS Form 8937, which explains how the deal affects shareholders’ basis. Keep a copy: your broker’s cost basis report is sometimes wrong after a merger, and RSU shares are a common source of errors.

Between announcement and closing, the stock usually trades near, but a little below, the offer price. That gap reflects the risk that the deal falls apart or is delayed by regulators. You can sell in the market during this window, which lets you choose your own sale date instead of accepting the closing date.

What happens to unvested RSUs, options and ESPP in an acquisition

Unvested RSUs

The plan document and merger agreement usually allow one of four outcomes:

• Assumed or converted. The buyer replaces your RSUs with its own RSUs of equal value, on the same vesting schedule. This is a common outcome when the buyer is a large public company.
• Cashed out. Unvested units are converted into a right to cash, often still paid on the original vesting dates.
• Accelerated. Some or all units vest early. “Single-trigger” acceleration happens at closing. “Double-trigger” acceleration happens only if you are terminated without cause (or leave for good reason) within a set period after closing.
• Cancelled. Less common for RSUs at public companies, but possible if the plan allows it.

Conversion itself is not a taxable event. Tax on RSUs still happens when shares are delivered at vest, as wages, with federal supplemental withholding often at the optional 22% flat rate for 2026 (employers may use the aggregate method instead), and a mandatory 37% on supplemental wages over $1,000,000 in the year (IRS Publication 15). If you want the mechanics of that, see do RSUs count as income.

Stock options

Vested options are often cashed out for the spread (deal price minus strike price) or converted into buyer options with an adjusted strike and share count. Options with a strike above the deal price (“underwater”) may be cancelled for nothing. A cash-out of nonqualified options is taxed as wages. For incentive stock options, the special tax treatment requires holding the shares at least two years from grant and one year from exercise (IRC §422(a)(1)). A cash deal that closes before those dates turns the sale into a disqualifying disposition, taxed partly as ordinary income. Our piece on tax on cashless exercise walks through the spread math.

ESPP

Many plans end the current offering period early and make a final purchase shortly before closing. Those shares, and any older ESPP shares you still hold, are then treated like other shares in the deal. In a cash deal, shares that do not meet the ESPP holding periods are a disqualifying disposition, with part of the gain taxed as wages.

Worked example: a California biotech family in a cash acquisition

Assumptions (illustrative): married filing jointly, $950,000 of W-2 income, federal and California taxable income of about $900,000 before the deal. One spouse’s employer is acquired for $50.00 per share in cash. The spouse holds 4,000 vested shares from RSUs, with a cost basis of $30.00 per share (the value at vest, already taxed as wages), and 3,000 unvested RSUs. The buyer trades at $125.00.

Vested shares. Gain is ($50 minus $30) times 4,000 = $80,000.

• If held more than one year: federal long-term capital gains rate of 20% (2026 MFJ taxable income above $613,700) = $16,000, plus 3.8% Net Investment Income Tax = $3,040. California taxes capital gains as ordinary income; at the 11.3% bracket (2025 schedule, the latest published) that is $9,040. Total: about $28,080.
• If held one year or less: federal 37% (2026 MFJ taxable income above $768,700) = $29,600, plus $3,040 NIIT, plus $9,040 California. Total: about $41,680.

The $13,600 difference comes only from the holding period. Shares from RSUs that vested within the 12 months before closing are short-term. The family cannot move the closing date, but knowing which lots are short-term helps them plan other gains and losses for the year.

Unvested RSUs. If the buyer assumes them at the deal ratio ($50 divided by $125 = 0.4), the 3,000 units become 1,200 buyer RSUs worth $150,000 at today’s price. Nothing is taxed at conversion. Each future vest is wages at the buyer’s price on that date. The family’s risk has shifted from one company to another, not gone away.

Same family in Washington. Washington has no wage income tax in 2026, and its 7% capital gains tax applies only to long-term gains above a standard deduction ($278,000 for 2025, the latest published figure). With no other gains that year, the $80,000 gain would owe no Washington tax. Note that Washington has enacted a 9.9% income tax on AGI over $1,000,000 starting in 2028 (WA Department of Revenue), so a deal closing in 2028 or later needs a fresh look.

The decisions a family actually makes before closing

1. Sell before closing or wait. Selling in the market locks in a price slightly below the offer, but removes deal risk and lets you choose the sale date (and, if closing may slip into the next year, sometimes the tax year). Waiting captures the full price if the deal closes.
2. What to do with the new buyer shares. In a stock deal, you may end up with a large position in the buyer plus new buyer RSUs. That is a fresh concentration decision, not a default to accept. Our investment risk management work often starts here.
3. Estimated taxes. A cash deal produces a lump sum with no withholding on the capital gain. At these income levels, an estimated payment may be needed to avoid underpayment penalties, depending on what was already withheld and paid for the year.
4. Your job. Read the change-in-control and severance terms. Double-trigger acceleration only helps if you know the window and the definition of “good reason”.
5. Cash in hand. A cash deal can turn a concentrated position into a large, liquid sum. Decide in advance how much goes to the emergency reserve, the 529s, the mortgage or a diversified portfolio, so it is not decided by default.

Common mistakes after an acquisition

• Trusting the broker’s basis figure. RSU shares are sometimes reported with a basis of zero or the wrong amount, which can lead to paying tax twice on income already taxed as wages.
• Assuming the tax is withheld. Cash-deal proceeds for shares you own are not wages, so nothing is withheld for the gain.
• Treating converted RSUs as “the same”. The new grant follows the buyer’s plan, trading windows and blackout rules.
• Ignoring tender offers. Some acquisitions start with a tender offer rather than a merger vote. See how a tender offer works.

What to write down for your kids

An acquisition is one of the few times a family makes a large, visible money decision in a short window. Your children will see the result (a paid-off mortgage, a funded 529, or nothing obvious at all) but not the reasoning. A one-page Decision Memo that records the deal terms, what you sold or held, why, and what you would do differently gives them the reasoning. As Kevin Luu puts it: “Your kids don’t need to inherit your portfolio. They need to inherit your decision-making process.”

Frequently asked questions

Do I have to sell my shares when my company is acquired?

No action is usually needed. In a merger, your shares are converted automatically at closing into cash, buyer shares or both, and the proceeds land in your brokerage account. You can choose to sell earlier in the market. A tender offer is different: you typically must tender your shares by a deadline, although a follow-on merger often converts shares that were not tendered.

What happens to my stock price after an acquisition is announced?

In a cash deal, the target’s price usually jumps close to the offer price and then trades just below it until closing. In a stock deal, the target’s price tends to move with the buyer’s share price times the exchange ratio. A wide gap between the market price and the offer often signals that investors see real risk the deal will not close.

Is a stock-for-stock merger always tax-free?

Not always. It must meet the reorganization rules in IRC §368, which the companies structure for and disclose in the deal documents. If it qualifies, you generally owe nothing until you sell the new shares, apart from tax on any cash received, including cash for fractional shares. If it does not qualify, the exchange is taxed like a sale.

Next step

If your employer has announced a deal, pull three documents this week: the merger proxy or prospectus, your equity plan’s change-in-control section, and your broker’s lot-level cost basis report. With those in hand, the tax and concentration choices become concrete. If you would like a second set of eyes, a Tax & Wealth Assessment through our tax management service covers exactly this.

This is general education, and the right move depends on the family’s full picture.

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