Complexation reactions of bivalent metal ions and ethyl-2,3-dioxobutyrate-2p-bromophenylhydrazone (EDOB-2p-BPH) have been studied potentiometrically in 50% (v/v) ethanol/water medium at different ionic strengths with respect to NaClO4 and at different temperatures and their stability constants determined. The method of Bjerrum and Calvin as modified by Irving and Rossotti has been used to determine then andpL values. Smin values which have the same significance as ‘T2’ have also been calculated. The thermodynamic stability constants and standard free energy change (ΔG) have also been calculated. ΔG values are negative in all cases indicating that the reactions are spontaneous. The ligand field stabilization energy (δH) has also been calculated for the 3d transition metals.