Abstract: High yields of phthalaldehydic acid from .alpha.,.alpha.,.alpha.,.alpha.',.alpha.'-pentachloro-o-xylene can be obtained in an excellent degree of purity if a mixture of said pentachloroxylene and phthalaldehydic acid in water is heated to a temperature between about 80.degree. and about 180.degree. C., and preferably at the boiling temperature of the reaction mixture. The mutual ratio of the reactants is not critical, and may vary within wide ranges, although it is usually preferred to have such a ratio between the said pentachloroxylene and phthalaldehydic acid that for each part by weight of the former at least 0.1 part by weight of the latter is present. Practically, there is no upper limit to the amount of phthalaldehydic acid which may be present, even to a large molecular excess.