The present invention relates to a process for the preparation of a mixture of aspartic acid diethoxy succinate and an amino acid derivative of the general formula (I) wherein n is 1-10, m is 0 or 1, and R is hydrogen or an alkali metal or alkaline earth metal ion, comprising reacting maleate with diethanol amine under alkaline conditions in the presence of a lanthanoid catalyst to form aspartic acid diethoxy succinate followed by adding aspartic acid which reacts with unreacted maleate to form imino disuccinic acid, i.e. an amino acid derivative of formula (I) wherein m is 0, or by adding a diamine derivative of the general formula (II) NH2(CH2)nNH2 wherein n is as defined above, which reacts with unreacted maleate to form an amino acid derivative of formula (I) wherein m is 1 and n is as defined above.