The stability constants (ln K) of the Co(II), Ni(II), Cu(II), Ag(I), Cd(II) and Hg(II) complexes with 4’-morpholinoacetophenone 4-phenyl-3-thiosemicarbazone were determined using a spectrophotometric method at different temperatures (25, 30, 35 and 40 +/- 0.1 degrees C) and at different ionic strengths (0.05, 0.10 and 0.20 mol dm(-3)) in 60% (v/v) aqueous dimethylformamide. Plots of thermodynamic stability constants at zero ionic strength (ln K-0) versus T-1 gave linear curves, and Delta H theta and Delta S theta were also calculated from these plots. Moreover, the changes in free energy for each metal-ligand system were calculated from the equation Delta G theta = -RT ln K-0.