The quenching effect of NO, O-2, CO, and N2O on the photoluminescence of Mo6+/SiO2 has been studied at room temperature as a function of gas pressure. Nonlinear plots of the relative photoluminescence intensity I-0/I (I-0 is the initial intensity under vacuum) vs. pressure of the quenching gases were rationalized assuming that only adsorbed molecules efficiently quench the (Mo5+-O-)double dagger excited state and that the fraction of adsorbed quenching molecules can be determined from the classical Langmuir-type adsorption isotherm. The ratio of the quenching rate constants for NO and CO calculated from the computer best fits of the experimental I-0/I pressure dependence is in agreement with earlier data on the kinetics of the photocatalytic reduction of NO by carbon monoxide on Mo6+/SiO2.