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Abstract A simple and sensitive spectrophotometric method for the determination of sulphamethoxazole in pure as well as in dosage form is described. The method is based on the reaction of sulphamethoxazole as electron donor with o-chloranil as electron acceptor to form violet colored charge transfer complex having maximum absorption band at 539 nm. Under the optimized reaction conditions, Beer’s law was obeyed in the range of 2-60 μg ml-1 with molar absorptivity 3.675×103 L mol−1cm−1. The limits of detection (LOD) and limit of quantitation (LOQ) were 0.371 and 1.236 μg ml-1 respectively. The accuracy and precision of the method were satisfactory; the average recovery % was 100.95 % and values of relative standard deviations ≤1.06 %. The stoichiometry of the reaction was studied, and the reaction mechanism was postulated. The proposed method was successfully applied to the determination of sulphamethoxazole in its pharmaceutical tablets and syrup with good accuracy and precisions. Keywords: Charge transfer; Spectrophotometry; o-chloranil; sulphamethoxazole