A conservative variant of discrete ordinate model was used to solve the radiation transport equation. The model prediction was used to assess the effect of wall reflectivity, catalyst loading, and phase function parameter on the light intensity distribution in an annular heterogeneous photocatalytic reactor. For relatively low catalyst loadings, the wall reflectivity strongly influenced the light intensity distribution. However, for an optically thick medium, the wall reflectivity had very little or no effect. The volume-average light intensity distribution decreased rather sharply with the catalyst loading and an opposite trend was obtained for the local volumetric rate of energy absorption (LVREA). However, after the initial sharp increase, the LVREA appeared independent of catalyst loading. For nonreflecting reactor walls, phase function parameter did not show much influence. However, for the specularly reflecting reactor walls and optically thin mediums, a moderate dependency on the phase function parameter was observed. (C) 2004 American Institute of Chemical Engineers.