A framework to determine the quantum efficiency eta of a photoreaction in a porous layer of a photocatalyst is presented, The procedure relies on a model of the photoproduct diffusion in the porous structure of the photocatalyst. The model incorporates a position-dependent source term mirroring the Light intensity profile in the layer and an effective diffusion coefficient D-eff It allows for a simultaneous determination of eta as well as of D-eff. The method is applied to the photosynthesis of CH4 from gaseous H2O and CO2 at the solid/gas interface of a porous layer of TiO2 (Degussa P25). A value of eta = (8.79 +/- 0.79) x 10(-4) is found for the formation of CH4. The effective diffusion coefficient for the photoproduct CHS is D-eff = (5.64 +/- 2.51) x 10(-10) cm(2) s(-1). This value is much too low to be explained by classical Knudsen diffusion in the layers with an experimentally determined porosity phi = 0.58. We suggest that the TiO2 layers contain a large number of unconnected pores separated by walls of densely packed microcrystals almost impenetrable for gases such as CH4 Whether processes other than diffusion are also responsible for the small value of D-eff is not clear at present.