Process for the preparation of 1,4-diketopyrrolo[3,4-c]pyrroles of the formula I ##STR1## in which R.sub.1 and R.sub.2 independently of one another are phenyl, biphenyl or naphthyl radicals or aromatic O-heterocyclic, S-heterocyclic or N-heterocyclic radicals, by reacting 1 mole of a dicyclohexyl succinate, dialkyl succinate, monoalkylmonophenyl or diphenyl succinate, alkyl in the succinic acid ester radical being C.sub.1 -C.sub.18 alkyl and phenyl being phenyl which is unsubstituted or substituted by one or two halogen atoms or one or two C.sub.1 -C.sub.6 alkyl or C.sub.1 -C.sub.6 alkoxy groups, with 2 moles of a nitrile of the formula II or III or with 1 mole of a nitrile of the formula II and 1 mole of a nitrile of the formula III ##STR2## in which R.sub.1 and R.sub.2 are as defined above, in an inert organic solvent, in the presence of an alkali metal or an alkali metal alcoholate, as a strong base, and at an elevated temperature to give an alkali metal salt of the formula IV ##STR3## in which M is an alkali metal, and subsequently liberating a compound of the formula I by protolysis of a compound of the above formula IV, which comprises treating, in stages, a compound of the above formula IV in at least two portions, as a protolysing agent, with water, with an inorganic and/or organic acid, with a mixture of water and an organic solvent, or with a mixture of an inorganic or organic acid and water and/or at least one organic solvent, and reacting the mixture at a temperature from 20.degree. to 150.degree. C. after the addition of each of the portions.