We have studied the effect of substituent R on the rate and mechanism of thermal decomposition of copper(II) complexes with L-α-amino acids anions H2NCH(R)COO−. The homolytic decomposition rate increases in the series of CuL2 complexes with alanine, glycine, valine, isoleucine, leucine, phenylalanine, and serine. Intramolecular redox formation of ·+CuGly cation-radical upon reduction of Cu(II) to Cu(I) and oxidation of H2NCH2COO− into H2NCH2COO· carboxylamine radical is the primary stage of heat-induced decomposition.