A process for preparing phthalides by I. reducing phthalic acid or phthalic acid derivatives, where the carboxyl groups may be replaced by units which can be derived from the carboxyl groups by a condensation reaction and where one or more of the hydrogens of the o-phenylene unit may be substituted by inert radicals, at a cathode in a undivided electrolytic cell and dissolved in an electrolyte, II. discharging the electrolyte from the electrolytic cell when the reaction has proceeded to the stage where the molar ratio (M), formed by the proportion of phthalide and the sum of the proportions of phthalide and phthalic acid of phthalic acid derivatives in the electrolyte, is from 0.8:1 to 0.995:1, and III. crystallizing the phthalides from the electrolyte and removing them from the mother liquor, if appropriate after distillative work-up of the electrolyte is described.