Cobalt(II), Ni(II), Cu(II) and Hg(II) form 1:2 (metal-ligand) complexes with 2-methyl-indole-3-carboxaldehyde 4-phenyl-3-thiosemicarbazone. The stability constants of these complexes in 60% (v/v) aqueous DMF were determined at different ionic strengths (0.10, 0.20 and 0.40 M KNO3) and of 25, 30, 35 and 40 degrees C (+/-0.1 degrees C) using a spectrophotometric method. From these constants, the thermodynamic stability constants and thermodynamic parameters Delta G(theta), Delta H-theta and Delta S-theta were calculated.