A process is disclosed for preparing 3-pyrroline-2-carboxylic acid derivatives of formula (I), in which R1 stands for H, C¿1?-C6-alkyl, benzyl, benzyl substituted at the phenyl radical, allyloxycarbonyl, C1-C6-alkyloxycarbonyl, benzyloxycarbonyl, in which the benzyl radical can be substituted by OCH3 radicals, or for C1-C4-alkylcarbonyl; or R?1¿ stands for an amino acid radical which can be alkylated or acylated at the nitrogen and is linked by the C-terminal; and R2 stands for OH, C¿1?-C4-alkyloxy, benzyloxy or an N?3R4¿ group, in which R?3 and R4¿ represent independently from each other H, C¿1?-C4-alkyl, benzyl, phenyl or pyridyl, in which the aromatic compounds in R?3 and R4¿ can be substituted by up to three identical or different substituents selected from the group composed of methyl, methoxy, hydroxy, cyano or halogen. According to this process, the sulphonic acid radical is eliminated by means of a base from a compound having the formula (II), in which R?1 and R2¿ have the above meanings and R5 stands for C¿1?-C6-alkyl, benzyl, trifluoromethyl, naphthyl or phenyl optionally substituted by radicals from the group composed of methyl, nitro and halogen.