A simple, rapid, accurate and sensitive spectrophotometric method has been developed for the quantitative determination of sulphamethoxazole (SMX) in both pure form and its pharmaceutical preparations. The proposed method is based on the diazotization of SMX with sodium nitrite in hydrochloric acid medium to form a diazonium salt, which is coupled with γ-resorsolic acid (2,6-dihydroxybenzoic acid) in alkaline medium of sodium hydroxide to form a stable and watersoluble azo dye exhibited maximum absorption at 432 nm against reagent blank. Beer's law is obeyed over the concentration range of 5 to 300 µg of SMX /20 mL (0.25-15 ppm) with a good determination coefficient (R 2 =0.9984) and apparent molar absorptivity 3.34×10 4 l.mol-1 .cm-1. The limit of detection (LOD) and limit of quantification (LOQ) are 0.0584 and 0.1752 µg mL −1 , respectively. The recoveries and relative standard deviations in tablets and oral suspension formulations are found as 96.6 to 99.04% and 1.6 to 3.0%, respectively, depending on the concentration level. The method is suitable for the determination of SMX in the presence of other excipients that are usually present in dosage forms. The composition of the resulting product has also been worked out and it is found to be (1:2)SMX: γ-resorsolic acid. This procedure is applied successfully to the analysis of SMX in pharmaceutical preparations (tablets and oral suspension) without prior separation but with acceptable errors.