Trichloroisocyanuric acid composition

Water sanitizing compositions are disclosed which are comprised of a mixture of trichloroisocyanuric acid and an alkaline earth metal solubility promoting additive selected from the group consisting of the bicarbonate, carbonate, hydroxide, oxide and mixtures thereof of said alkaline earth metal. The compositions have a molar ratio of said additive to trichloroisocyanuric acid of from about 0.3:1 to about 3:1 and a moisture content of less than about 0.2 percent by weight of the compositions. The novel compositions rapidly dissolve in water bodies such as those found in swimming pools, commodes, automatic dishwashers and the like to quickly provide a sanitizing amount of available chlorine for sanitizing the water.