The square-wave voltammetric behaviour of cysteine and saccharin was studied at a static mercury drop electrode at pH 7.4 in the presence of Cu(II) ions. In the presence of excess Cu(II), cysteine exhibited three reduction peaks for Hg(SR)2 (-0.086 V), free Cu (II) (-0.190 V) and Cu(I) SR (-0.698 V), respectively. Saccharin produced a catalytic hydrogen peak at -1.762 V. In the presence of Cu(II), saccharin gave a new peak (-0.508 V), corresponding to the reduction of Cu(II)-saccharinate, which in the presence of cysteine formed a mixed ligand complex (-0.612 V), CuL2A2 (L = saccharin and A = cysteine). The peak potentials and currents of the obtained complexes were dependent on the ligand concentration and accumulation time. The stoichiometries and overall stability constants of these complexes were determined by Lingane's method (voltammetrically) and Job's method (spectrophotometrically). The mixed ligand complex in the molar ratio 1:2:2 (log β=33.35) turned out to be very much stronger than the 1:1 Cu(I)SR (log β=21.64) and 1:2 Cu(II)-saccharinate (log β = 16.68) complexes. Formation of a mixed ligand complex can be considered as a type of synergism. (C) 2000 Elsevier Science S.S.