A rapid and sensitive electroanalytical method has been developed for measuring free chlorine in water by using a wax-impregnated carbon electrode. This method can differentiate free chlorine from other combined chlorine species. Sodium dichloroisocyanurate, an active chlorine compound widely used in swimming pool disinfection, was chosen as the model system for the study. Reagents, chronoamperometry, iodometry, sensitivity and precision, temperature, rate of reaction, pH, and dissociation are discussed.