A process for preparing perylene-3,4:9,10-tetracarboxylic diimides of the general formula I ##STR00001## comprising dimerizing a naphthalene-1,8-dicarboximide of the formula II ##STR00002## in a reaction medium consisting essentially of an apolar aprotic organic solvent and an alkali metal base and subsequently reoxidizing the resulting alkali metal salt of the leuco form of the perylene-3,4:9,10-tetracarboxylic diimide in the presence of a polar solvent.