To obtain a peracid that is useful as an oxidizing agent in the production epoxy, quinone or lactone compounds by reaction of an ester with hydrogen peroxide in the presence of an acid catalyst.

An ester from acetic, propionic and butyric acid and an alcohol of 1-6 carbon atoms, for example, an ester of the formula: R1-COO-R2 (R1 is a 1-3C alkyl, amino or the like; R2 is a 1-6C alkyl, amino) is oxidized with hydrogen peroxide diluted with water or an organic solvent in the presence of an acid catalyst as sulfuric acid or a compound bearing sulfuric acid group in an amount of 0.001-50 wt.% based on the weight of the ester thereby giving the objective peracid. As it is present in an ester, this peroxide can be readily separated from the product system. In addition the problem on corrosion of the production plant can be minimized because no carboxylic acid is used in the process.