Abstract A kinetic method for the determination of sulphonamides by the continuous addition of reagent (CAR) technique is reported. The method involves the formation of an azo dye between 1-naphthol and a diazonium salt, which is in turn obtained on reaction between the free primary aryl amine of the sulphonamide and sodium nitrite in a weak acid medium. The reaction is monitored via the initial rate of change of the absorbance of the azo dye formed at 470 nm, which is proportional to the sulphonamide concentration. Practical problems associated with this type of reaction for the determination of sulphonamides were readily avoided by using the CAR technique. Sulphonamides can be determined in the range 3 × 10 −6 −3 × 10 −5 M with a relative standard deviation of about 1% and a sampling frequency of 25 h −1 (triplicate runs). The proposed method is simple and rapid and insensitive to the presence of excipients; it proved useful for the determination of sulphonamides in pharmaceutical samples.