SummaryA comparative study of the autoxidation of methyl oleate, methyl 9,10‐dideuteroöleate, methyl stearolate, and stearolic acid has re‐emphasized the complexity of the autoxidation phenomenon. The autoxidation of the acetylenic compounds indicates the importance of peroxidation of the active methylene group as compared to the addition of oxygen at the unsaturated linkages. The importance of olefinic hydrogens and the role of the double bond with respect to the induction periods and possible addition reactions has been brought out by the studies on the autoxidation of the deuteroöleate. Consequently it appears that during autoxidation of methyl oleate, oxidative attack is initiated at the olefinic position and then at α‐methylenic position and that water formation occurs shortly thereafter. At least some of the hydrogen of the water formed during autoxidation comes from the olefinic hydrogens.It appears that most of the non‐aqueous volatile cleavage products arising from a rupture of a carbon to carbon bond are peroxidic in character. The nature of these volatile peroxides appears different from several commercially available organic peroxides since they are capable of oxidizing methyl oleate in a manner to yield the high melting form of dihydroxystearic acid whereas peracids and some peroxides under similar conditions give only the low melting isomer.