AbstractIn the presence of an immobilized lipase fromCandida antacrtica (Novozym 435R) fatty acids are converted to peroxy acids by the reaction with hydrogen peroxide. In a similar reaction, fatty acid esters are perhydrolyzed to peroxy acids. Unsaturated fatty acid esters subsequently epoxidize themselves, and in this way epoxidized plant oils can be prepared with good yields (rapeseed oil 91%, sunflower oil 88%, linseed oil 80%). The hydrolysis of the plant oil to mono‐ and diglycerides can be suppressed by the addition of a small amount of free fatty acids. Rapeseed oil methyl ester can also be epoxidized; the conversion of C=C‐bonds is 95%, and the composition of the epoxy fatty acid methyl esters corresponds to the composition of the unsaturated methyl esters in the substrate.