ISO (Incentive Stock Option)
An Incentive Stock Option (ISO) is a type of employer-granted stock option that, when the applicable statutory requirements are met, allows the employee to purchase company shares at a predetermined exercise price without recognizing ordinary income at the time of exercise. ISOs are governed by IRC Section 422 and offer favorable tax treatment compared to non-qualified stock options, though they are subject to the alternative minimum tax and numerous qualifying conditions.
ISOs are exclusively available to employees of the granting corporation (not independent contractors or non-employee directors) and must comply with a strict set of statutory requirements to retain their preferential tax status. The option must be granted under a plan approved by shareholders and must specify the total number of shares reserved and the employees eligible to receive grants. The exercise price must be at least equal to the fair market value of the stock on the grant date (110% of fair market value for shareholders owning more than 10% of the company). The option must be exercised within 10 years of the grant date (5 years for 10%-or-more shareholders).
The defining tax advantage of an ISO is that no ordinary income is recognized at grant or at exercise under the regular income tax system. The employee pays no income tax when the option is granted and no ordinary income tax when they exercise the option and purchase the shares. Instead, if the employee satisfies the qualifying disposition holding period — holding the shares for at least two years from the grant date and at least one year from the exercise date — the entire gain from exercise price to ultimate sale price is treated as a long-term capital gain when the shares are sold.
However, the spread between the exercise price and fair market value at exercise is an adjustment item for the alternative minimum tax (AMT) under IRC Section 56(b)(3). For employees at companies with rapidly appreciating stock, exercising a large block of ISOs in a single calendar year can trigger a substantial AMT liability, even if the employee has not yet sold the shares and received any cash. This was a significant problem for employees at technology companies during the dot-com era, and it remains a material planning concern for ISO holders at high-growth private and public companies.
If the holding period requirements are not met — for example, the employee sells the shares within one year of exercise — a disqualifying disposition occurs, and the spread at exercise becomes ordinary income taxable at the employee's marginal rate, similar to the treatment of a non-qualified stock option. Any additional appreciation beyond the spread at exercise retains short-term or long-term capital gain characterization depending on the holding period.
ISOs are subject to an annual $100,000 limit: the aggregate fair market value of stock (measured on the grant date) underlying ISOs that first become exercisable in any calendar year cannot exceed $100,000 per employee. Options exceeding this limit are treated as non-qualified stock options to the extent the excess arises. Companies issuing ISOs are not entitled to a compensation deduction unless a disqualifying disposition occurs, which is the flip side of the favorable employee tax treatment.