Proton-ligand dissociation and metal-ligand stability constants of 3-(p-Tolylsulphonamido)rhodanine (TSR) with some transition metal ions were calculated potentiometrically in 0.1 M KC1 and 20% (v/v) ethanol-water mixture. The order of stability constants was found to be: Th4+>UO2+2>Gd3+>Ce3+>La3+>Mn2+<Co2+<Ni2+<Cu2+>Zn2+The dissociation constantspKHof TSR and the stability constantslog Kof their complexes were determined at different temperatures (303, 308 and 318 K). The corresponding thermodynamic parameters (ΔG, ΔH and ΔS) were derived and discussed. The dissociation process is non-spontaneous, endothermic and entropically unfavourable. The formation of the metal complexes have been found to be spontaneous, exothermic or endothermic (depend on the metal) and entropically favourable. The stoichiometries of these complexes were determined spectrophotometrically and indicated the formation of 1:1 and 1:2 (metal:ligand) complexes.