 This paper reports the results of an investigation carried out on the interaction between Zn(II) and some hydroxocarboxylic ligands (L), namely: malate (Mala2−), tartrate (Tar2−) and citrate (Cit3−). The stability constants of the mono- and polynuclear ZniHjLk(2i+j−kz) species were determined by potentiometric measurements at T = 298.15 K in different ionic media, NaCl(aq) and NaNO3(aq), at different ionic strengths: I = 0.15 mol dm−3 (NaCl(aq)) and 1.00 mol dm−3 (NaCl(aq) and NaNO3(aq)). As a further contribution to the study of the formation of mixed species, that are of particular importance in fluids that hyperaccumulated metal ions, the ternary interactions between two different metals and a ligand (Zn2+/Sn2+/hydroxocarboxylic ligands) were also investigated. The formation of the hetero-metal complexes produces an enhancement of the stability of the complexes with respect to the same homo-metal ones, shifting significantly the pH of precipitation of the sparingly soluble species and increasing the mobility and bioavailability of the species. From the analysis of experimental data at I = 1.00 mol dm−3 in NaCl(aq) and NaNO3(aq), it results that the stability constants in NaCl(aq) are significantly higher than the ones in NaNO3(aq). The higher stability of the species in NaCl(aq) can be explained in terms of ion pairs formation with chloride ion. As an example, for the hetero-metals/Mala/chloride system, the following stability constants, on the basis of the general reaction: mixed-complex + Cl− = mixed-complex/Cl (quaternary species), we obtained: log K1111 = 0.48, log K1121 = 0.36, log K112-11 = 0.82 for ZnSnMalaCl, ZnSn(Mala)2Cl, and ZnSn(Mala)2OHCl, respectively. The sequestering ability of the three ligands toward Zn2+ was evaluated, at different experimental conditions, by means of the empirical parameter, pL0.5. The effect of the chloride anion on the stability of various species and on the sequestration of the metal was also evaluated.