Abstract: A stable, free-flowing sodium dichloroisocyanurate dihydrate powder is prepared by spray drying an aqueous solution or slurry of sodium dichloroisocyanurate to form a particulate product having a moisture content from about 12.0 to about 20.0% by weight and subjecting this product to specific stabilizing and cooling after-treatment steps in a plurality of fluidized and/or vibrating beds. During the stabilizing step the product is dried to the final moisture level, if necessary, and the water of hydration is uniformly distributed and firmly bound to the particulate product.