The kinetics of the oxidation of long-chain unsaturated compounds when spread as monolayers on KMnO4 solutions has been determined from measurements of the changes in area with time. The effects of number of double bonds (unconjugated),of chain length, and of permanganate concentration were studied, as well as the behaviour of the dihydroxy compounds believed to be the first stage of the oxidation process. The velocity constant has been found to increase markedly for each additional double bond, and to be independent of chain length, thus providing a quantitative measure of degree of unsaturation in unconjugated long-chain compounds. Mixed films of unsaturated and dihydroxy acids have been studied and the results used to allow for deviation from additivity of areas in the reacting systems. On this basis the overall area-time curve, which goes through a maximum, has been quantitatively accounted for.