A highly sensitive, simple and accurate spectrophotometric method has been developed for quantitative determination of sulfamethoxazole(SMX) in both pure form and pharmaceutical preparations. In this method SMX is diazotized with equimolar of sodium nitrite(NaNO 2) in acid medium of hydrochloric acid to form diazonium ion , which is reacted with 2,4,6-trihydroxybenzoic acid in alkaline medium of NaOH to form a yellow water soluble azo dye that has absorption maximum at 416 nm versus reagent blank. Beer's law is obeyed over the concentration range 0.2-16 µg. ml-1 with an excellent determination coefficient (r 2 = 0.9996) and molar absorptivity 1.84×10 4 l.mol-1 .cm-1. The recoveries are obtained in the range of 97.8-99.8% and the relative standard deviation is better than ±0.23%. The stoichiometry of the resulting azo dye has been also worked out and it is found to be 1:1 SMX: 2,4,6-trihydroxybenzoic acid. This method has been applied successfully for the determination of SMX in pharmaceutical preparations (tablets and oral suspension).