Gas-phase O2 quenching of toluene laser-induced fluorescence (LIF) is studied at 300-650 K in a N2/O2 bath gas of 1-bar total pressure with O2 partial pressures?400 mbar. With increasing vibrational excitation of the laser-excited toluene, intramol. decay becomes faster, resulting in a decreasing relative strength of collisional quenching by O2. Stern-Volmer plots are nonlinear for temps. >500 K in the case of 266-nm excitation and at all temps. for 248-nm excitation. This is attributed to the onset of internal conversion from specific vibrational levels. A photophys. model is developed that describes the exptl. data and predicts toluene LIF signal strengths for higher O partial pressures. For practical application, O2 quenching is not the dominant deexcitation process for engine-related temp. and pressure conditions, and application of the popular fuel-air ratio LIF (FARLIF) concept leads to erroneous signal interpretation.