Exposure of luminescent n-type porous Si to gaseous molecular oxygen results in reversible quenching of the visible photoluminescence associated with this material. Steady-state and time-resolved photoluminescence quenching follow a dynamic Stern-Volmer model. From the Stern-Volmer analysis, the quenching rate constant, k(q), was found to be 26 +/- 9 Torr(-1) s(-1). The rate constant for quenching is not strongly dependent on the chemical composition of the surface. Hydride-, deuteride-, or oxide-terminated surfaces all display similar quenching rate constants. Quenching is attributed to electron transfer from the luminescent chromophore in porous Si to an O-2 molecule weakly chemisorbed to a surface defect. In parallel with the reversible quenching process but on a much longer time scale (minutes to hours depending upon light intensity), porous Si samples also slowly photooxidize. Both the intensity (measured at steady state) and lifetime (measured by nanosecond-pulsed laser excitation) of photoluminescence decrease as the surface oxide layer grows, approaching a constant value after several hours of O-2 exposure. The mechanism of photochemical oxidation is proposed to involve the same photogenerated O-2 species produced during quenching.