Proton-ligand formation constants of isatin-3-thiosemicarbazone (HIT) ; N-acetylisatin-3-thiosemicarbazone (HAIT) and 5-(p-nitrobenzoyl)-1,2,4-triazino[5,6-b]indole-3-thione (HBITr) ligands and their corresponding metal-ligand formation constants with Mn2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, UO22+ and Th2+ ions were determined pH-metrically at 10, 20, 30 and 40°C in 75 %(v/v) ethanol–water. The thermodynamic parameters (DG, DH and DS) were also evaluated. It was found that both log K1 and –DH1, for HIT and HAIT-complexes are somewhat larger than log K2and –DH2, indicating a change in the dentate character of these ligands from tridentate (ONN-donors) in 1:1 chelates to bidentate (ON-donors) in 1:2; M:L chelates. In contrast, the values of log K2 and – DH2 for HBITr-complexes are somewhat larger than log K1 and –DH1, indicating a strong trans-effect for the second coordination. The dissociation process is non-spontaneous, endothermic and entropically unfavourable while the complexation process is spontaneous, exothermic and entropically favourable. The thermodynamic parameters were separated into their electrostatic (el) and non-electrostatic (non) constituents.