The temperature dependence (300 – 700 K) of toluene absorption and fluorescence were studied with varying oxygen concentrations (2-35%) at 1 bar total pressure with laser excitation at 248 and 266 nm. Contrary to the popular FAR-LIF assumption, the toluene-LIF signal is not directly proportional to the fuel-air ratio for many conditions relevant to reciprocating IC engines. With increasing temperature, a higher oxygen partial pressure is required to justify the FAR-LIF assumption. The required minimum oxygen pressure becomes unrealistic (> 5 bar) for approx. T > 670 K at 266 nm excitation and for T > 625 K at 248-nm excitation.