Taxes After a Startup Acquisition or Tender Offer
Cash-out events, RSU vesting, QSBS, and withholding on tender offers — what founders and employees owe when shares actually convert to money.
A tender offer or acquisition is the moment paper wealth becomes cash — and the IRS shows up. Whether you're selling secondaries in a tender, accepting merger consideration, or exercising options ahead of a close, the tax treatment depends on what you hold, how long you've held it, and how the deal is structured.
Withholding at source is almost never enough for California or New York residents selling at a large gain. Plan for supplemental withholding gaps and estimated payments in the quarter of the sale.
Deal terms vary wildly. This walkthrough covers common structures for VC-backed startups — not every M&A tax issue. You'll want a CPA who has seen actual closing statements, not just cap table theory.
Tender offer vs acquisition — different tax timing
A tender offer usually lets you sell existing shares to a buyer (often the company or a new investor). You're selling stock you already own. Gain is generally long-term capital gain if you held the shares more than a year — short-term if not.
An acquisition may pay cash, stock of the acquirer, or both. Cash is taxable at closing. Acquirer stock may be taxable too (unless it's a qualifying reorganization with specific holding requirements — get counsel's memo on this).
Unvested options and RSUs are often cashed out or assumed by the acquirer. Unvested RSUs that accelerate on change-of-control are taxed as ordinary income at vest/acceleration — not capital gain.
How each equity type gets taxed at exit
ISOs: If you exercised more than a year ago and sell more than a year after exercise (and also meet the 2-year-from-grant test), the gain can be long-term capital gain (AMT may have applied at exercise). Same-day exercise-and-sell in a tender is a disqualifying disposition — the spread is ordinary income on your W-2, but employers generally do not withhold federal income tax or FICA on that ISO spread the way they do for NSOs.
NSOs: The spread at exercise is ordinary income (W-2 or 1099-NEC). Sale price above exercise price is capital gain. Many tender processes require exercise first, which triggers that ordinary income layer immediately.
RSUs: Taxed as wages when they vest or when restrictions lapse. If acceleration happens at acquisition, expect a big W-2 line item. Supplemental withholding at 22% federal leaves a gap for higher earners.
Restricted stock with 83(b): If you filed the election, you've already paid tax on the purchase price. Sale proceeds are mostly capital gain. No 83(b)? You may have been taxed at vest already — basis depends on what was reported.
QSBS — the exclusion worth verifying early
Section 1202 QSBS rules now depend on issuance date. Stock issued on or before July 4, 2025: exclude up to $10M (or 10× basis) after a full 5-year hold, with a $50M gross-assets test at issuance. Stock issued after that date (OBBBA): tiered exclusion of 50%/75%/100% at 3/4/5 years, a $15M per-issuer cap (or 10× basis), and a $75M gross-assets test. Either way you still need a C corp, original issuance, and a qualified trade or business.
Don't assume you qualify because someone said 'QSBS' in a board deck. Pull your stock purchase agreement date, verify the company's asset test at issuance, and check whether any redemptions or conversions broke the chain.
If you qualify, Form 8949 and Schedule D still get filed — you report the sale and then exclude the gain with proper documentation. For older stock, missing the five-year hold is still a cliff; for post-July 4, 2025 stock, a 4-year hold can still deliver a 75% exclusion.
Withholding and estimated tax planning
Companies typically withhold on tender proceeds as supplemental wages (22% federal flat, plus state). A $2M gain doesn't stay at 22% marginal — you'll owe 37% federal plus state on much of it.
Make estimated tax payments (Form 1040-ES) in the quarter of the sale if withholding won't cover 90% of current-year tax or 110% of prior year. Underpayment penalties add up fast on seven-figure gains.
If you're exercising NSOs to sell in the tender, you'll have both ordinary income and capital gain on the same transaction. Model both layers before you check the box to participate.
Forms you'll see after the close
Form 1099-B or a substitute statement from the broker handling the tender — reports proceeds and (hopefully) basis.
Form W-2 with a large supplemental wage line if RSUs accelerated or NSO spreads hit payroll.
Form 8949 and Schedule D on your 1040 for stock sales. Form 6251 if ISO exercises trigger AMT.
State returns in every state where you lived while earning the equity (not just where you live at sale) — California especially cares about work performed while a resident.
Checklist before you sign the tender letter
☐ Confirm holding periods for LTCG vs ordinary income
☐ Model tax on exercise-and-sell vs sell existing shares
☐ Verify QSBS eligibility and five-year clock if relevant
☐ Ask what withholding rate applies; budget for the gap
☐ Check whether signing triggers single-trigger acceleration on unvested equity
☐ Get a CPA to review the letter and cap table before the election deadline — tender windows are short
Key takeaways
- Tender proceeds aren't all capital gain — NSO spreads and accelerated RSUs hit as ordinary income first.
- 22% supplemental withholding rarely covers the actual bill on large exits; plan estimated payments.
- QSBS can exclude substantial gain ($10M or $15M cap depending on issuance date), but eligibility is factual — verify before you count on it.
- Model exercise-and-sell economics before the tender window closes; you can't undo the election.
This guide is for general education only and is not tax, legal, or accounting advice. Rules change, and your facts matter. Talk to a qualified professional before filing or making equity decisions.
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