The oxidative action of hypochlorite on indigocarmine is investigated. Hypochlorite dissociates indigocarmine to produce isatin-5-monosulphonic acid, with a stoichiometry of 2:1. The probable mechanism is proposed. The suitability of reaction between hypochlorite and indigocarmine as an indicator reaction for the determination of high levels of hypochlorite in synthetic and commercial samples is investigated. A simple and reliable kinetic-analytical method is developed for the determination of hypochlorite ion at alkaline pH, based on its oxidizing activity.