The oxidative stability of phytosterols during microwave heating was evaluated. Two different model systems (a solid film made with a phytosterol mixture (PSF) and a liquid mixture of phytosterols and triolein (1:100, PS+TAG)) were heated for 1.5, 3, 6, 12, 20 and 30 min at 1000 W. PS degraded faster when they were microwaved alone than in presence of TAG, following a first order kinetic model. Up to 6 min, no phytosterol oxidation products (POPs) were generated in both systems. At 12 min of heating, POPs content reached a higher level in PSF (90.96 µg/mg phytosterols) than in PS+TAG (22.66 µg/mg phytosterols), but after 30-min treatment, the opposite trend was observed. 7-keto derivates were the most abundant POPs in both systems. The extent of phytosterol degradation depends on both heating time and the surrounding medium, which can impact the quality and safety of the food product destined to microwave heating/cooking.