One of the remaining challenges in application of heterogeneous photocatalysis for treatment of air streams containing dilute VOCs is to design a cost-effective photocatalytic reactor that simultaneously allows efficient contact of the contaminated air and solid catalyst while uniformly irradiating the solid catalyst with light. A pseudohomogeneous model was developed to study effects of system parameters on process performance for a gas - solid lamp-in-tube annular-photocatalytic-oxidation (PCO) reactor in which the annular space is filled with photocatalyst-coated packing. In this model the flow field is assumed to be uniform and radial diffusion negligible. Homogeneous reactions are neglected. Heterogeneous reaction rates follow Langmnir-Hinshelwood-Houghen-Watson kinetics with rate parameters extracted from independent experiments. A I-D "two-flux" incidence submodel is used to account for the radial UV light distribution throughout the reactor annular space. This submodel requires knowledge of the UV lamp radiant emittance, the optical characteristics of the catalytic thin-film coating, and the UV irradiance at the outer wall of the reactor and contains only a single adjustable parameter - the mean free path between photon - catalyst interactions. The model was validated with experimental performance measurements for destruction of acetone and isopropyl alcohol in a bench-scale photoreactor. The validated model can be used to predict the optimum catalyst film thickness for given reactor dimensions, packing shape and size, and VOC abatement problem.