Do I owe taxes when my ISOs vest?
No. Vesting does not trigger a tax event. The tax clock starts at exercise, not at vesting.
Stock Options
Incentive Stock Options
The tax-advantaged grant that rewards patience if you navigate the AMT.
Key mechanics
- Who can receive
- Employees only
- Tax at exercise
- AMT preference item (spread)
- AMT rates (2026)
- 26% up to $244,500; 28% above
- Tax at sale
- LTCG if holding periods met
- LTCG holding period
- 1 yr from exercise + 2 yrs from grant
- Annual ISO vesting limit
- $100K FMV per calendar year
How it works
An ISO is a contractual right to purchase shares at a price set on the grant date: the strike price. That price must equal the fair market value of the stock at grant, as determined by a 409A valuation. You don't own shares until you exercise. Exercising means paying the strike price for each share. The spread, the difference between the current FMV and your strike price, is what holds value. Until exercise, you hold an option, not equity.
AMT and the phantom tax problem
Under the regular federal tax system, exercising an ISO does not trigger a taxable event. However, the 'bargain element' or spread, the difference between the FMV on the exercise date and the strike price, must be added back to your income for the Alternative Minimum Tax calculation. The AMT applies a 26% rate to AMT-taxable income up to $244,500, with a 28% rate above that threshold (2026 figures). Because this AMT liability is triggered before you sell any shares, a large ISO exercise can create an immense, immediate tax bill on paper gains that remain entirely illiquid, commonly called 'phantom tax.'
Qualifying vs. disqualifying dispositions
If you hold shares for at least one year from exercise and two years from grant date, your sale is a 'qualifying disposition' and gains are taxed at long-term capital gains rates. Selling before either threshold is a 'disqualifying disposition': the spread at exercise is recharacterized and taxed at ordinary income rates in the year of sale, eliminating the tax benefit of the ISO structure. The $100K annual ISO vesting cap means options above that value in a single calendar year automatically convert to NSOs, a detail often overlooked in large grants.
Common questions
What is the ISO AMT trap?
When you exercise ISOs, the spread between your strike price and the current FMV is an AMT preference item. If that spread is large, you may owe Alternative Minimum Tax in the exercise year even though you haven't sold any shares, meaning you'd need cash to pay a tax bill on paper gains. This is why exercising ISOs early, when the spread is small, is a common strategy.
What happens to my ISOs if I leave the company?
Most plans give you 90 days after termination to exercise your vested ISOs. After 90 days, they either expire or convert to NSOs depending on plan terms. After conversion to NSOs, you lose the ISO tax treatment. The spread at exercise becomes ordinary income.
What is a qualifying disposition?
A qualifying disposition means you sold the shares at least one year after exercise AND at least two years after the original grant date. This qualifies your gain for long-term capital gains rates. Selling before either threshold is a disqualifying disposition and triggers ordinary income tax on the spread.
Related guides
Stock Options
Non-Qualified Stock Options
Broader eligibility, simpler taxes, but ordinary income at exercise.
Strategy
Early Exercise & 83(b) Election
Exercise before vesting to start the capital gains clock, with a 30-day filing deadline.
Foundation
409A Valuations
The independent appraisal that sets your strike price and anchors every tax calculation.
Exercise Financing
Non-Recourse Financing
Specialty funds cover your exercise cost. Zero personal liability if the company fails.
Providers for this path
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Sources
Put it into practice
Use the scenario modeler to see how Incentive Stock Options mechanics play out with your specific grant, or join the waitlist for deeper, personalized equity intelligence.