The total acidic products from the performic acid oxidation of the ozonide of methyl oleate formed in methanol may be esterified directly in a few hours with 2,2-dimethoxypropane. The ester concentrations are adequate for the determination of the positional isomers of monoethylenic fatty acids directly from the reaction mixture, using a hydrogen flame ionization gas–liquid chromatography detector. Dimethyl sulfoxide was not required to prevent the breakdown of 2,2-dimethoxypropane under the conditions employed.