A first-derivative FTIR spectrometric method has been developed for the simultaneous determination of sulphamethoxazole (SMX) and trimethoprim (TMP) in their binary dosage formulations. The procedure is based on extraction of active components in a solution of 30% v/v acetone in chloroform, filtration and measurement of the amplitudes of the first derivative spectrum at 1138 and 1166 cm−1for TMP and SMX respectively. Good linearity, accuracy and precision were found for this method. The relative standard deviation for TMP and SMX were found to be 1.5 and 1.3% in a commercial Co-trimoxazole tablet, respectively (n=5).