Oxidation products of stigmasterol were characterised by their polarity and molecular size using solid phase extraction (SPE) and high-performance size exclusion chromatography (HPSEC) methods. Monomeric oxides were studied further by GC-MS and GC-FID. The focus was on identifying and quantifying non-polar and mid-polar monomeric oxides after SPE fractionation. Commercial stigmasterol was subjected to 180 degrees C up to 3 h. Six oxidation products were identified by GC-MS in the non-polar and mid-polar monomeric fractions; all appeared during the first hour of heating. Quantification by GC-FID showed an increase in the non-polar and mid-polar oxidation products during the heating time, and their amounts reached values of 6.1 and 47.0 g/kg of commercial stigmasterol, respectively. Polar oxidation products commonly measured reached a value of 193 g/kg after 1 h of heating, while after 3 h of heating their concentration was only 164 g/kg. Since as much as 550 g/kg of stigmasterol was decomposed, the monomeric products explained only partly the stigmasterol loss. Dimeric and polymeric products contributed to 165 g/kg of the loss showing the importance of polymerisation reactions at 180 degrees C.