<b>Colorimetric Determination of Stability Constant of Trimethoprim Cu II Complex at Different Temperatures by Continuous Variation Method</b> Trimethoprim is an antibiotic that is mainly used for the treatment of bladder infections. Continuous variation method was applied in the calculation of stability constant of trimethoprim Cu II complex depending on the theoretical explanation of the stoichiometry. The formation of Cu II complex with trimethoprim was studied colorimetrically at an absorption maximum of 480 nm at 25, 30, 35 and 40 oC. The data showed that Cu II and trimethoprim combine in the molar ratio of 1 1 at pH 7.4 with ionic strength maintained using 0.1M KNO3. Calculated stability constants values were 6.00 x 104, 3.80 x 104, 2.61 x 104 and 1.89 x 104 at 25, 30, 35 and 40 oC respectively. Calculated GO for the complex were 2.73 x 104, 2.66 x 104, 2.60 x 104 and 2.56 x 104 at 25, 30, 35 and 40 oC respectively. The stoichiometry, stability constant and Gibbs free energy results suggested that trimethoprim used in the study is a potent chelating agent and can be an efficient antidote in the therapy of Cu II overload or poisoning. Trimethoprim, copper, complex, stability constant 252-256 Issue-5 Volume-4 O. V. Ikpeazu | I. E. Otuokere | K. K. Igwe