Effectiveness factors for porous catalysts have been determined for photocatalytic reactions characterized by integer order and by generalized Langmuir-Hinshelwood/Hougen-Watson rate expressions. The geometry of interest is a planar porous membrane that is illuminated from either the front or the rear by a planar light source. Effectiveness factors for these systems depend not only on Thiele moduli defined herein but also on the product of the effective membrane thickness and the optical absorption coefficient of the porous membrane.