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Unsaturated lipids are prone to oxidize and polymerize when they are exposed to oxygen, heat, and light. Polymerization of the lipids changes the properties of the media, and it is therefore considered a deterioration process. In the present study, the polymerization of steryl esters was investigated. The effects of acyl moiety and temperature were examined by oxidizing neat cholesteryl stearate, oleate, and linoleate and free cholesterol at two temperatures (100 and 140°C). The formed reaction products were characterized by SEC-RI and silver coordination ion MS (Ag+-CIS-MS). Moreover, the secondary oxidation products of sterol were determined by GC-FID/MS. High-molecular size compounds could be detected in the steryl esters after the heat treatment, and the presence of oligomers up to pentamers and hexamers could be indicated. In cholesteryl stearate, 79 mg/g of oligomers were measured after 3 days of heating at 100°C, while in cholesteryl oleate and cholesteryl linoleate, the contents were 569 and 809 mg/g, respectively. As the temperature was increased to 140°C, the rate of polymerization increased. After 1 day of heating, 428 mg/g of oligomers were measured in cholesteryl stearate, 552 mg/g in cholesteryl oleate, and 679 mg/g in cholesteryl linoleate. Free cholesterol remained unaltered at 100°C, but at 140°C, 420 mg/g of oligomers were measured after 1 day of heating. In conclusion, polymerization was a considerable reaction as part of the autoxidation of steryl esters. Moreover, the reaction competed with the formation of secondary oxidation products. Practical applications: Several types of food products are fortified with plant sterols because of their ability to lower serum cholesterol levels. Therefore, it is important to study the oxidation behavior of sterols and their conjugates. The subject is relevant since radicals and oxidation products of sterols may form during the manufacture of the preparations, thus leading to higher contents of oxides and possibly oligomers in the final food products. Polymerization of lipids could affect also the cholesterol-lowering efficiency of plant sterols in fortified food products. The obtained results improve the knowledge on how the primary and secondary oxidation products react into more stable products that may accumulate during prolonged heating. [ABSTRACT FROM AUTHOR]