Photoinduced visible light emission from porous silicon can be reversibly quenched by a wide variety of chemical species. The growth of a thin layer of oxide on the porous silicon surface disrupts the quenching ability of most species, narrowing down the number of quenchers to include primarily those which act as Bronsted bases. Electron paramagnetic resonance spectroscopy, infrared spectroscopy, photoluminescence data, and surface chemistry suggest a quenching mechanism which involves the extraction of a nonspecifically attached proton in the oxide layer upon exposure to base. This proton is loosely affiliated with a surface defect of the P-b type. This defect serves as a hole trap in the absence of a proton providing a nonradiative relaxation pathway. However, when a proton is present in the oxide layer, Coulombic interactions force the hole trap into a state which falls below the bandgap, allowing for efficient radiative recombination of electron-hole pairs. The electron paramagnetic resonance spectroscopy data also demonstrate that there are at least two distinct mechanisms of luminescence quenching of porous silicon.