Photoluminescence from porous silicon can be quenched reversibly by a variety of molecular species. Quenching pathways for chemically incorporated surface species, physisorbed species (that undergo no net chemical transformation), and for electron donating reagents have all been identified. For systems involving charge transfer quenching, the concentration dependence typically follows a Stern-Volmer type of relationship, with the more easily oxidized molecules producing the largest Stern-Volmer slopes (most efficient quenching). The slope of the Stern-Volmer plot is dependent upon the particular wavelength that the Stern-Volmer data are obtained from. Data interpretation is complicated by the fact that porous Si shows a wavelength dependent emission lifetime, although when this is taken into account the data can be qualitatively described within the context of a driving-force dependent quenching model. A simple model for understanding the various photoluminescence quenching phenomena observed with porous Si is presented, involving an ensemble of emissive states with energy dependent lifetimes and Stern-Volmer quenching behavior. The model adequately accounts for both red and blue spectral shifts that have been observed upon photoluminescence quenching.