Abstract: Phthalide, one intermediate in the oxidation of o-xylene to phthalic acid or its anhydride, is unique in that it is the only intermediate oxidation product which cannot be economically removed from said anhydride by fractional distillation or crystallization. According to the present invention, impure phthalic anhydride containing phthalide is heated to a temperature of at least 200.degree. C. in the presence of a catalytic amount of an alkali metal hydroxide having a molecular weight above 40. By such heat treatment of impure phthalic anhydride its phthalide content can be decreased to at least 0.1 weight percent and even to below the analytically detectable concentration of 0.001 weight percent (10 ppm by weight).