The adsorption and photocatalytic oxidation of formaldehyde on the pure TiO2 under dry and humid conditions were studied by in situ diffuse reflectance Fourier transform infrared spectroscopy. It was found that the formaldehyde molecules can be adsorbed on the hydroxyl groups on the TiO2 surface via hydrogen bonding. With UV irradiation, the adsorbed formaldehyde rapidly converts to the formate species even on the pure TiO2 at room temperature and atmospheric pressure. In the dry environment, the superoxide radical anion O-2(-)center dot, formed by adsorbed oxygen reacting with electrons, is suggested to play an important role in the formaldehyde oxidation. The introduction of water vapor provides a large amount of water and hydroxyl groups on the catalyst surface. Oxidation of water and hydroxyl groups by the photogenerated holes produces very active OH center dot radicals, which take part in the redox reactions and improve significantly the mineralization rate of formaldehyde on the TiO2 due to its high redox potential.