Abstract: A process for preparing perylene-3,4:9,10-tetracarboxylic diimides of the general formula I where R is C1-C30-alkyl whose carbon chain may be interrupted by one or more —O— moieties and/or which may be substituted by one or more substituents selected from the group consisting of C5-C8-cycloalkyl (which may be substituted by one or more C1-C6-alkyl substituents), phenyl or phenyl-C1-C6-alkyl (which may each be substituted by one or more C1-C18-alkyl and/or C1-C6-alkoxy substituents), —OCOR1, —N(R1)2, —SO2NH2, —SO2N(R1)2, —CON(R1)2 and —COOR1; C5-C8-cycloalkyl whose carbon skeleton may be interrupted by one or more moieties selected from the group consisting of —O—, —S— and —NR2- and/or which may be substituted by one or more C1-C6-alkyl substituents; phenyl, phenyl-C1-C6-alkyl, naphthyl or hetaryl, which may each be substituted by one or more substituents selected from the group consisting of C1-C18-alkyl, C1-C6-alkoxy, phenylazo, naphthylazo, pyridylazo, pyrimidylazo, cyano, —N(R1)2, —CON(R1)2 and —COO