In this study, two simple and sensitive spectrophotometric procedures were developed for determination of the stepwise equilibrium constants of 1:1 and 2:1 adduct formation of Ni(dtp)2 with antazoline in toluene. One is based on the measurement of absorbances of series of solutions, which contained constant total concentration of Ni(dtp)2 and antazoline at 455 nm, and the straight line plot of Rose-Drago equation in this special case. This method was applied for the determination of the formation constant and molar absorptivity of 1:1 adduct and they were found to be 3073±40 and 308.5, respectively. The other is based on the spectrophotometric measurement of the solutions in constant Ni(dtp)2 concentration with increasing antazoline concentration. From the slope and the intercept values of the derived equation, the equilibrium constant and the molar absorptivity of 2:1 adduct were calculated and they were found to be 152±2 and 51.9±2.6, respectively. Keywords :