Spectrofluorometric techniques have been used to study the effect of added xenon on the singlet and triplet yields of monofluorobenzene vapor excited by 2670, 2520, and 2470 Å radiation. For each wavelength examined, the fluorescent yield was found to decrease with increasing xenon pressure and plots of 1/Фp vs.Pxe are linear. The sensitized emission of biacetyl technique was used to determine triplet yields. The effect of xenon on the emission yield of biacetyl excited directly at 4358 Å was also examined and it was found that xenon reduced the phosphorescent yield of biacetyl. Plots of 1/Φp vs. Pxe were found to be linear, for all cases of sensitized as well as directly excited emission. At 2470 Å both the fluorescence yield and the triplet yield initially increase with increasing Xe pressure, but when the Xe pressure exceeds about 30 Torr the behavior at this wavelength is the same as it is at the other two wavelengths.The data suggest that xenon enhances intersystem crossover in both monofluorobenzene (FB) and biacetyl (BiA). The important process in the former is 1FB + Xe → 3FB + Xe, while in the latter is 3BiA + Xe → BiA + Xe. A mechanism consistent with the experimental observation is presented.