Sixty grams (0.4 mole) of piperonal (Note 1) and 1.5 l. of water are placed in a 5-l. flask fitted with an efficient mechanical stirrer. The flask is placed on a steam bath, heated to 70–80°, and the stirrer started (Note 2). A solution of 90 g. (0.56 mole) of potassium permanganate in 1.8 l. of water is allowed to flow into the emulsion of piperonal and water over a period of forty to forty-five minutes (Note 3). The stirring and heating are continued for an hour longer, at the end of which time the permanganate is reduced. Enough 10 per cent potassium hydroxide solution is added to make the solution alkaline. The mixture is filtered while hot, and the manganese dioxide is washed with three 200-cc. portions of hot water. The combined filtrate and washings are cooled. At this point any unreacted piperonal that separates must be filtered (Note 4). The solution is now acidified with hydrochloric acid, the acid being added until no further precipitate forms. The resulting piperonylic acid is filtered, washed with cold water until free of chlorides, and dried. The yield is 60–64 g. (90–96 per cent of the theoretical amount) of a colorless product melting at 224–225° (corr.). For most purposes the crude material is pure enough, but it may be crystallized from ten times its weight of 95 per cent ethyl alcohol, yielding 52–56 g. (78–84 per cent of the theoretical amount) of needles melting at 227–228° (corr.).