The tax on Employee Stock Purchase Plans (ESPP) has two components: the difference between the offering price and the fair market value (FMV) of the stock is treated as employment income and the difference between the FMV and the selling price is treated as capital gains or losses.

For example, let’s say that shares in your employer ABC Inc. was offered to you at a price of $10 on September 15th and on that day, the stock closed at $12. The company beat expectations and when the markets opened for trading the next day, you sold the stock at $13. The $2 benefit ($12 – $10) is treated as employment income and typically taxed at your marginal tax rate and the $1 gained when selling the stock is treated as capital gains and only 50% of it is subject to taxes. The portion considered as employment benefit is tracked by the employer and included in Box 40 of your T4 slip. It is your responsibility to track the capital gains portion of ESPP profits and report it in Schedule 3 of your tax return.

The peculiarities of the tax treatment of ESPPs underline the risks inherent in holding on to company stock. If you were offered shares at $2 when the company stock is trading at $20, you are on hook for income tax on the difference, irrespective of what you finally sold the stock for. If the stock tanked and you finally sold at $2, you’ll have capital losses of $18 that can only be used to offset capital gains elsewhere in your portfolio, not the original employment benefit.