6-Hydroxy-2-isopropyl-4-methyl pyrimidine(HIMP) could be decomposed by photocatalysis. This photocatalytic degradation reaction obeyed first order kinetics. The degradation rate constants were high between pH 4 and pH 6. In acidic or basic conditions, HIMP and titania surface have such the same charges that HIMP cannot adsorb on titania surface. As a result, HIMP was thought to be decomposed more slowly in acidic and basic conditions. It was suggested that the hydroxyl radical attacked pyrimidine ring directly and mineralized HIMP completely.