Abstract The photolysis half-life of aqueous chlorine in clear sky, summer noon sunlit (47°N) water of pH 8 is 12 min when measured at the surface. It increases with decreasing pH due to the decreasing ratio of OCl −/HOCl to 60 min at pH 5. The pseudo-first-order rate constant for the photolysis of HOCl becomes 2 × 10 −4 s −1 and that of OCl − 1.2 × 10 −3 s −1. The variation of the rate of photolysis with depth was calculated for water columns exhibiting different light absorption coefficients by taking into account that, for both HOCl and OCl −, the most effective wavelength for photolysis in sunlight is approx. 330 nm. These results show that in water treatment, chlorine photolysis should be minimized whenever possible by operating at low pH, sun shielding or night-time addition of chlorine or avoiding storage in shallow reservoirs. The rate of chlorine photolysis controls the formation of OH radical which acts as a secondary highly reactive photooxidant (see Part II). On u.v. (255 nm) irradiation both HOCl and OCl − photolyze at comparable rates and slowly enough that chlorine depletion will not occur during the time of irradiation typical in u.v. disinfection. Photolysis can also contribute to the depletion of chlorine in atmospheric waters whenever chlorine is formed by (slow) ozonation of chloride.