The effect of 5% sitosterol on the autoxidation of tristearin at 120°C was investigated. It was found that sitosterol caused a two-fold increase in the process rate; this effect was unchanged at peroxide concentrations of up to 350 mEq kg−-1. It was established that introduction of 5% sitosterol considerably increased the concentration of oxidised products in tristearin (from 0.5 to 6.5%) after oxidation at 120°C for 7h. The degree of oxidation of sitosterol in the system was found to be 56%. The structure and composition of oxidised products in the two lipid systems (tristearin, and tristearin+5% sitosterol) were determined. Conclusions were drawn concerning the effect of the lipid medium on the composition and structure of the products of sitosterol autoxidation.