A distributed parameter model for photocatalytic oxidation of air contaminants in monolith reactors is presented. Heat and mass balance equations for monolith structure are combined with a model of irradiation from a light source and a kinetic model for photon adsorption and chemical reaction to describe the processes of heat, mass and photon transfer within the system and the heterogeneous chemistry at the catalyst surface. The model accounts for interaction between light and matter at the catalyst surface, convective and interphase gas-solid heat and mass transfer, reaction at the catalyst surface and heat conduction within the solid structure. Together with detailed axial profile of temperature and conversion in the gas phase and at the catalyst surface under different operating conditions (inlet gas temperature, composition and flow rate, light source power, monolith geometry), the model provides the distribution of photon flux along the channels and allows the discrimination between thermal and pure photonic effect on the overall rate of conversion.