In this study, we examine theoretically some of the factors that influence the spectral dependencies of the quantum yield, Φ, of photochemical processes on the surface of nano-/microparticulates of wide-band-gap metal-oxide photocatalysts. The approach taken is based on solving the steady-state ((dn(t)/dt) = 0) continuity equation (eq 7) for a one-dimensional plate with dimensions x = 0-d irradiated on both sides and considering the spatial nonuniformity of photogeneration of charge carriers in the bulk of the solids and their limited probability of diffusion toward the particulate surface. This spatial nonuniformity of charge carrier photogeneration depends on whether the solid photocatalyst is photoexcited in the intrinsic or extrinsic regions or whether only the surface is photoexcited. Essential conditions required to observe similarities between the spectral dependencies of the quantum yield of surface processes and the absorption spectrum are (i) the relative values of the absorption coefficient α (cm^-1) and (ii) the complexity of the absorption spectrum. Spectral selectivity of photocatalysts appears to depend on the different probabilities that electrons and holes reach the photocatalyst surface which are the result of differences in their mobilities and lifetimes (i.e., different diffusion lengths of the carriers).