The stability constant of Zonisamide with trivalent rare earth metal ions Ce3+, Pr3+, Nd3+, Pm3+ and Gd3+ using a pH metric titration technique in aqueous medium at 25°C temperature and at an ionic strength of 0.2M NaClO4 were studied. The method of Calvin and Bjerrum as adopted by Irving and Rossotti has been employed to determine metal-ligand stability constant (logK) values. The trend in the formation constants follows the order: Pr3+ < Ce3+ < Nd3+ < Pm3+ < Gd3+