Four TiO2-based materials, named A, B, C and D, are used to investigate the influence of water vapor on the gas-solid adsorption and heterogeneous photocatalytic oxidation of gaseous methyl ethyl ketone (MEK). Two of the photocatalysts (A and B) are constituted of powdered TiO2 deposited onto two different supports (ordinary glass and non-woven cellulose fibers). The other ones (C and D) are composed of a thin film of TiO2 coated on glass substrates. The effect of water vapor on MEK initial conversion rates is studied for the four photocatalyfic materials using the Langmuir-Hinshelwood model at the initial time. On the concentrations range where the model hypotheses are verified, adsorption constants K and kinetics constants k are calculated for experiments under both dry and humid atmosphere. When the relative humidity is increased, the evolution of these constants shows that water vapor acts differently depending on the form of deposited TiO2 (powder and film). (c) 2005 Elsevier B.V. All rights reserved.