Nowadays, with the increase in the thermal insulation of buildings, indoor air quality (IAQ) has deteriorated, particularly because of the presence of volatile organic compounds (VOC). To improve IAQ photocatalytic processes can be used. Photocatalytic reactions can be broken down into three steps: adsorption of pollutants, chemical reaction, and desorption of water, carbon dioxide and by-products. In this work, the accessibility of the pollutants to the reactive sites of a commercial TiO2-photocatalyst is studied. Firstly, adsorption mechanisms are investigated through toluene adsorption isotherms in batch reactors. In humid air conditions (relative humidity of 50% at 24 degrees C), the classical adsorption models cannot be applied. Consequently, a new model, called the "Langmuir-multi", is built. It fits the obtained experimental data properly. Adsorption equilibrium constants are calculated based on this new model. To understand adsorption mechanisms better, adsorption isotherms are also performed in dry air conditions with toluene and in humid air with acetone and heptane. It is observed that water vapor plays a major role. Secondly, photocatalytic reactions are carried out with toluene, acetone and heptane in humid air. The kinetic curves are well-represented by the Langmuir-Hinshelwood (L-H) equation so that reaction and equilibrium constants can be assessed. The L-H equilibrium constant appears to depend on the type of pollutant and on the affinity between the pollutant and the photocatalyst surface. No correlation is found between the equilibrium constant and light intensity. It is also shown that the calculated L-H equilibrium constants are not equal to those previously obtained in dark conditions and in batch reactors. (C) 2011 Elsevier B.V. All rights reserved.