The stability constant , Kf , for the complexation of copper (II) ion with 4 -Amino - 3- methyl -1 , 2 , 4 - triazol - 5 -thione ( AMTT ) in 0 , 20 , 40 , 60 and 80 ( V / V ) % ethanol-water mixtures were determined conductometrically at different temperatures . Stability constant of resulting 1:1 complexes were being larger by increasing of temperature and ethanol percent. Stability constants of complexes vary inversely with dielectric constant of solvents .The enthalpy and entropy of complexation were determined from the temperature dependence of the formation constant . In all cases , the complexation were found to be enthalpy unstabilized but entropy stabilized . ΔG° of the studied complexes were evaluated at 25°C using thermodynamic relations , the negative values of ΔG° means that the complexation process is spontaneously .