Sulfamethoxazole is an antibiotic that is used for the treatment of bacterial infections such as prostatitis, bronchitis and urinary tract infections. It is effective against gram negative and gram positive bacteria. Classical equation has been used in the calculation of stability constant of SulfamethoxazoleCu(II) complex depending on the theoretical explanation of the stoichiometry. The formation of Cu(II) complex with sulfamethoxazole was studied colorimetrically at an absorption maximum of 430 nm at 25, 30, 35 and 40 oC. The data showed that Cu(II) and sulfamethoxazole combine in the molar ratio of 1:2 at pH 7.4 with ionic strength maintained using 0.1M KNO3. Calculated stability constants values were 4.02 x 106 , 2.93 x 106 , 1.37 x 106 and 9.21 x 105 using continuous variation method. Calculated ∆GƟ for the complex were -3.77 x 104 , -3.75 x 104 , -3.62 x 104 and - 3.57 x 104 . The stoichiometry, stability constant and Gibbs free energy results suggested that sulfamethoxazole used in the study is a good chelating agent and can be an efficient antidote in the therapy of Cu(II) overload or poisoning

Keywords : Sulfamethoxazole, Copper, Complex, Stability Constant.