At the Umm Al Nar Power and Desalination Plant (Abu Dhabi, UAE), seawater is continuously chlorinated to achieve a residual chlorine content (RCC) of 0.4 to 0.45 ppm, enough to discourage marine organisms gaining access inside the distillers. Reference to the plant＇s logbooks reveals that more chlorine is injected into the sea than is necessary to attain the desired RCC. The difference depends on the hour of the day as well as the season of the year. These variations are correlated to changes in seawater temperature, solar radiation and biological activity in the sea. Laboratory experiments are conducted to measure the disappearance of RCC from sea and distilled waters in the dark, in diffused daylight and in direct sunlight at different temperatures. The rate constants and concentration variations under the various conditions are computed and discussed. Distinction is made between self-decomposition of the ClO- ion, on the one hand, and reactions with seawater organics, on the other. Light accelerates both reactions more than does rise in temperature. Simple equations are suggested for the calculation of the quantities of chlorine injected into the sea over the whole year, and good agreement with actual values is established. Two alternatives are presented to reduce the amounts of extra chlorine added to the sea. The first recommends injection of the gas in complete darkness deep inside the seawater intake pipe. This realizes a saving of 14% of the total chlorine consumption. The second alternative suggests chlorination in diffused daylight, which ensures a decrease in chlorine consumption of 25% during day time. Both approaches curtail the adverse effect of chlorine on the environment.