TiO2 sols prepared in acidic and basic medium were deposited into films by a spin coating method. Photodegradation experiments showed that photocatalytic activity of the films prepared from acidic sol was much higher than that from basic sol. It is identified that there are more chemisorbed species of CO2 on the surface of the TiO2 films from the basic sol than on the surface of the TiO2 films from the acidic sol. The chemisorbed species of CO2 reduce the concentration of active species such as hydroxyl group and bridging oxygen on surface of the TiO2 film and contribute to the formation of surface electron traps in the band gap which are detrimental to charge separation, thus lowering the photocatalytic activity. (C) 2004 Published by Elsevier B.V.