A process for preparing perylene-3,4-dicarboximides which bear a sterically demanding substituent on the imide nitrogen atom by reacting a perylene-3,4:9,10-tetracarboxylic dianhydride with a sterically hindered primary amine in a substantially anhydrous reaction medium, which comprises undertaking the reaction in the presence of a tertiary amine, of a solvent based on a cyclic imine or amide and of a Lewis acid as a catalyst.