The vapor phase kinetics of S-1 --> S-0 p-difluorobenzene (pDFB) fluorescence quenching by O-2 has been characterized over an O-2 pressure range spanning more than 4 orders of magnitude, ranging from the single-collision regime at less than one Torr to about 37000 Torr. pDFB was pumped to an S-1 level with epsilon(vib) = 3310 cm(-1). Non Stern-Volmer kinetics is observed. The standard Stern-Volmer model, for which the ratio of fluorescence intensity without and with added oxygen against O-2 pressure is linear with an intercept of unity, fits the data only for pressures <10 Torr. At O-2 pressures >3000 Torr, the quenching again becomes linear but with a much lower slope and higher intercept. The quenching rate constants for the low- and high-pressure regimes are 1.3 x 10(11) L mol(-1) s(-1) = 7.7 x 10(6) Torr(-1) s(-1) and 0.13 x 10(11) L mol(-1) s(-1) = 0.78 x 10(6) Torr(-1) s(-1), respectively. Less detailed studies showed that quenching from S-1 levels with epsilon(vib) = 3705 and 2887 cm(-1) has kinetics similar to that of the 3310 cm(-1) level. A proposed mechanism involving two quenching channels fits the data over the entire pressure range. While there are no data identifying the products of these channels, pDFB T-1 formation may be the rate-determining aspect of the high-pressure quenching. It is suggested that the formation of a pDFB.O-2 charge-transfer complex may be rate controlling at low pressures. Low-level ab initio calculations give a rather tight complex geometry with a ring-to-O-2 distance of 2.5 Angstrom, a dipole moment of 2.6 D, and a net charge transfer of 0.6 electrons. The bonding energy relative to separated pDFB(+) and O-2(-) was calculated to be 38000 cm(-1).