Mesoropous TiO₂ was prepared using esterification between poly ethylene glycol (PEG) and acetic acid, and was applied to the photocatalytic decomposition of 2-isopropyl-6-methyl-4-pyrimidinol (IMP), cyanide ion, and rhodamine B (RB) in order to study the effect of surface area and pore size on the activity. When catalysts had similar specific surface areas, the activity of photocatalytic decomposition of IMP was proportional to the pore size due to more facile diffusion into the pore. However, in case of cyanide which size is much smaller than the pore size of TiO₂, the activity was proportional to the specific surface area, while a very large size molecule of RB caused low activity due to difficulty in diffusion into the pore.