Thermodynamic proton-ligand stability constants of N, N′-di- p-tolyl-5-nitro-2-thiobarbituric acid (DTNTBA) and the thermodynamic metal ligand stability constants of complexes of DTNTBA with some bivalent metal ions were determined by potentiometric measurements in 75% (v/v) aqueous dioxan medium at different temperatures, ranging from 20 to 50°C. The order of stability constants was found to be Cu 2+ > Ni 2+ > Co 2+ > Zn 2+ > Cd 2+ > Mn 2+ > Mg 2+. The values of S min ( χ 2) also been calculated. The thermodynamic functions along with the error range for the stepwise complexation processes were calculated at 20°C (error range ±0.5°C). The process is predominantly an enthalpy-driven process.