The photocatalytic chemisorption of water on titanium dioxide (TiO2) thin films was studied by photocurrent decay experiments on the films exposed to water vapour. The TiO2 films were deposited on quartz substrates by sputtering of a high purity TiO2 target in a radio frequency magnetron discharge in argon-oxygen gas. In vacuum (10(-2) Torr), the films with good photocatalytic activity had large induced photocurrent intensity and long electrical charge carrier lifetime, when compared to the films with low photocatalytic activity. The photocurrent decay in good photocatalytic films exposed to water-argon gas mixture at atmospheric pressure depended strongly on the gas humidity, larger decay rates being observed at larger humidity values. Because of the oxygen chemisorption, the photocurrent decay rates were larger in air than in the same humidity water-argon gas. (c) 2004 Elsevier B.V. All rights reserved.