The complexation equilibria of 2-hydroxybenzamide (salicylamide, HL) with UO(2)(2+), Al(3+), Nd(3+), and Ni(2+) ions have been studied at 25 degrees C and in 1 mol.dm(-3) NaClO(4) ionic medium by potentiometric measurements with a glass electrode. The concentration of ligand (C,) was varied between (1.10(-3) and 15.10(-3)) mol.dm(-3), while the concentration of metal ions M(z+) (C(M)) ranged within (0.5.10(-3) and 10.10(-3)) mol.dm(-3). The ligand to metal concentration ratio was varied between 1 and 20. 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: UO(2)L(+), UO(2)L(2), UO(2)(HL)L(+), UO(2)(OH)L, (UO(2))(2)(OH)L(2)(+), (UO(2))(2)(OH)(2)(2+), and (UO(2))(3)(OH)(5)(+); AlL(2+), AlL(2+), Al(OH)L(2), AlOH(2+), Al(OH)(2)(+), and Al(3)(OH)(4)(5+); NdL(2+), NdL(2)(+), NdOH(2+), and Nd(2)OH(2)(4+); and NiL(+), Ni(OH)L, and NiOH(+). Equilibrium formation constants are given for the investigated ionic media as well as for the infinite dilution reference state, evaluated by the specific ion interaction theory (SIT).