The stability of Cu(II), Ag(I) and Hg(II) complexes of 2-methylindole-3-carboxaldehyde thiosemicarbazone have been determined using a spectrophotometric method in 50% (v/v) aqueous ethanol at different ionic strengths (0.12, 0.22 and 0.4 M) and at different temperatures (25, 30, 35 and 40 +/- 0.1 degrees C). For each metal-ligand system at each temperature, the thermodynamic stability constant (In K-theta) was obtained graphically at zero ionic strength from plots of In K versus ionic strength. The thermodynamic parameters were evaluated from plots of In K-theta versus 1/T.