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.