In this paper we present a two-dimensional (2-D) analysis of a narrow-slit, flat-plate, single-pass, flow-through photocatalytic reactor for air purification. The continuity equation for convection and diffusion in two dimensions, under un-steady-state conditions, was coupled with radiation field modeling and photocatalytic reaction kinetics to model the transient and steady-state behavior of the reactor. The model was applied to the photocatalytic oxidation of trichloroethylene (TCE) in humidified air streams under different experimental conditions. The kinetic parameters determined by a three-dimensional (3-D) computational fluid dynamics model of the reactor were used in the 2-D model simulations. Under the experimental conditions in the reactor, the 2-D model was shown to approximate closely the experimental results of the oxidation of TCE and to give predictions that approach very well those of the 3-D model. The 2-D model, when applicable, provides a simpler and less time-consuming approach than the 3-D model with computational times reduced by a significant amount.