The reduction of Sb(III) with Arginine and Leucine was investigated polarographically in aqueous medium. Sb(III) formed 1:1, 1:2 and 1:3 complex species with Arginine as well as Leucine. The stability constants of Sb(III) with Arginine and Leucine were measured by the method of DeFord and Hume. The reduction of the system in each case is quasireversible and diffusion controlled, involving three electrons. The thermodynamic parameters have been determined. The stability constants of these species at 300K for Sb(III) with arginine are logb1 = 2.00, logb2 = 4.54, logb3 = 6.67 and at 310 K are logb1 = 2.00, logb2 = 4.17, logb3 = 6.63 and thermodynamic parameters free energy (KCal mol–1), enthalpy (KCal mol–1) and entropy (Cal mol–1 deg–1) are -2.62, -42.55 and -0.13 (MX1), -5.96, -39.08 and -0.11 (MX2), -8.75, -42.30 and -0.11 (MX3) respectively. The stability constants of these species at 300K for Sb(III) with Leucine are logb1 = 1.97, logb2 = 4.43 logb3 = 6.54 and at 310 K are logb1 = 1.84, logb2 = 4.30, logb3 = 6.61 and thermodynamic parameters free energy (KCal mol–1), enthalpy (KCal mol–1) and entropy (Cal mol–1 deg–1) are -2.58, -39.74 and -0.12 (MX1), -5.81, -42.26 and -0.12 (MX2), -8.58, -43.01 and -0.09 (MX3), respectively. The Mathematical Mihailov?s method has also been applied for the comparison of stability constants values obtained by graphical method.