Potentiometric titration studies for the ligands (barbituric acid (BA), 5-nitrobarbituric acid (NBA), phenobarbital (PB) and 2-thiouracil (TU)) and their complexes (Zn2+, Cd2+, Hg2+, VO2+, Pd2+, Y3+, Ce3+ and Sm3+) were done in aqueous media and in 50% (v/v) dioxane-water at different temperatures (25-40°C). The pKai values of 5-nitrobarbituric acid are smaller than that of barbituric acid due to the strong electron withdrawing nitro group. The pKai values of phenobarbital and 2-thiouracil in 50% (v/v) dioxane-water media are mostly higher than that in aqueous media, due to lower ionizing power of the mixed solvent. All the VO2+ complexes are of the highest stability constants. The thermodynamic parameters of the ligands and their metal complexes were investigated. The enthalpy-entropy relationship is applied for the complexes where the isokinetic temperatures (β) were obtained. Majority of complexation reactions are entropy controlled.