Abstract: Photometic study on the charge transfer reaction between trimethoprim,the electron donor,and alizarin,the electron acceptor,was made,and based on this charge transfer reaction,a photometric method for determination of trimethoprim was proposed.In a mixed solution of ethanol and water (7+3),the charge transfer complex was formed between trimethoprim and allizatin with its absorption maximum at the wavelength 546 nm,and its mole ratio of 1 to 1.Apparent molar absorptivity (ε) was found to be 3.82×103L·mol-1·cm-1.Beer′s law was kept in the range within 70 mg·L-1.Precision of the method was tested at the concentration level of 40 mg·L-1 for 6 multiplicate determinations,and the value of RSD found was 1.0%.The proposed method was applied to the determination of trimethoprim in its tablets,giving results in consistency with the labeled value and with those found by HPLC.Recovery of the method was tested by standard addition method,and value of average recovery found was 98.1%.