A highly pure cyanuric acid produced by pyrolyzing a nitrogenous containing material capable of yielding cyanuric acid, such as urea, in an alkyl dinitrile solvent such as adiponitrile, at elevated temperatures, is disclosed. The overall process converts substantially all of the urea to cyanuric acid and cyanuric acid derivatives which are capable of being converted into trichloroisocyanuric acid and similar compounds, while allowing substantially complete recovery of the nitrile solvent for reuse.