A pyrimidine derivative, 2-isopropyl-6-methyl-4-pyrimidinol (IMP), could be completely decomposed following pseudo-first-order kinetics by photocatalytic reaction using TiO2. The effects of pH and surface modification of TiO2 with SiOx were studied. The degradation rate of IMP was fast in mild acidic condition (pH = 6.3) and slower in strong acidic (pH = 2.0) or basic (pH = 10.0) condition. The main reason of the slow IMP degradation was different in strong acidic and basic conditions. In strong acidic condition, the slow degradation rate was explained by electrostatic repulsion. On the other hand, a little formation of hydroxyl radicals was considered as a main reason in strong basic condition. The result was supported by the experiments using SiOx-loaded TiO2, which has a lower isoelectric point than pure TiO2. (C) 2003 Elsevier B.V. All rights reserved.