A process for preparing cyanuric acid by cooling an isocyanuric acid/ammonia gas mixture having a temperature of 300.degree. to 480.degree. C. to 200.degree.-300.degree. C., whereupon cyanuric acid is separated off and isolated at this temperature in an apparatus suitable for separating solids from gases, and unreacted isocyanuric acid/ammonia gas mixture is recycled or further processed.