The complexation equilibria of L-cystine, H(2)L, with the Cu(2+), Al(3+), and Fe(3+) ions have been studied at 25 degrees C and in 3 mol.dm(-3) NaClO(4) ionic medium by potentiometric measurements with a glass electrode. The concentrations of ligand (CL) and metal ions M(z+) (C(M)) were varied between (0.5.10(-3) and 5.10(-3)) mol.dm(-3), and the ligand-to-metal ratio was varied between 1 and 10 (1 <= C(L)/C(M) <= 10). The hydrogen ion concentration was decreased stepwise to the incipient precipitation of a basic salt of each metal. The experimental data have been explained with the formation of the complexes: CuL, CuL(2)(2-), Cu(HL)(2) and Cu(2)(HL)(3+); AlL(+) and AlL(2)(-); and FeL(+) and FeL(2)(-). Equilibrium formation constants are given for the investigated ionic medium as well as for the infinite dilution reference state, evaluated by the specific ion interaction theory (SIT).