Amino acid N-carboxyanhydrides (NCA), convenient monomer for polypeptide synthesis, are easily prepared in high purity as the result of N-carbamoyl amino acids (CAA) nitrosation by gaseous NOx (4:1 NO + O2 mixture, or NOCl) in toluene. Removal of polar side products is then efficiently carried out during subsequent work-up and crystallisation so that the resulting NCA obtained in good yield is suitable for controlled, primary amine-initiated polymerisation.