The invention relates to a process for the preparation of aminomalonic acid esters and optionally their salts by the catalytic hydrogenation of a substituted malonic acid ester in an inert solvent, and optionally by the addition of an acid to the hydrogenation mixture. The process comprises carrying out the hydrogenation in the presence of a solid dehydrating agent and a solvent or solvent mixture in which the aminomalonic acid ester salt is substantially insoluble.