This paper focuses on the adsorption of gaseous trichloroethylene, toluene and chlorobenzene on the photocatalyst TiO2 Degussa P25. An optimized EPICS (Equilibrium Partitioning In Closed Systems) methodology was used to study equilibrium partitioning. For the three compounds investigated, equilibrium adsorption was reached within 60 min of incubation. Adsorption isotherms, determined at a temperature (T) of 298.2 K and relative humidities (RH) of 0.0% and 57.8% were found to be linear (R-2 > 0.993, n = 5), indicating that no monolayer surface coverage was reached in the concentration interval studied (0.02 mg l(-1) less than or equal to C-g less than or equal to 10.45 mg l(-1)). Within the linear part of the isotherm, the influence of both relative humidity and temperature was investigated in a systematic way and discussed from a thermodynamic point of view. Data analysis resulted in a double linear regression In K = -(DeltaU(ads)R(-1)) T-1 + a RH + d(R-2 > 0.94, n = 13) for 22% less than or equal to RH less than or equal to 90% and 283 K less than or equal to T less than or equal to 313 K. The equilibrium adsorption coefficient K(1 g(-1)) represents the equilibrium concentration ratio C-solid (mg g(-1))/C-g (mg l(-1)) and DeltaU(ads) is the internal energy of adsorption (J mol(-1)). At RH = 0.0%, experimental K values were a factor 5-10 higher than those expected from the regression equation, indicating that another adsorption mechanism becomes important below monolayer surface coverage of TiO2 by water vapour molecules. Since surface interactions are of primary importance in photocatalytic reactions, this paper contributes to a better understanding of the basic mechanisms of TiO2 mediated heterogeneous photocatalysis and is an interesting tool for developing optimized mathematical models. (C) 2003 Elsevier Science Ltd. All rights reserved.