A spectrophotometric method has been developed for the microdetermination of sulfamethoxazole and trimethoprim drugs. The proposed method is based on the charge transfer (CT) complex formation of the drug with alizarin or quinalizarin in alkaline medium and measuring the developed absorbance at its maximum. The optimum conditions for maximum absorbance results are studied and Beer's law is obeyed in the ranges 10–130 and 10–190 μg/ml for sulfamethoxazole and trimethoprim respectively. For more accurate results, Ringbom optimum concentration ranges are 20–120 and 10–170 μg/ml respectively. The molar absorpitivity and Sandell sensitivity are also calculated. The proposed spectrophotometric method of analysis is as accurate as the USP method (USP XX. The National Formulary, NF XV, 1980 P. 925. Mack Easton, Pa., 1980) and is simpler than their official method. Applications of the suggested method to representative pharmaceutical sulfa drugs are presented and the validity of the method was assessed by applying the standard addition technique.