Toluene laser-induced fluorescence (LIF) was investigated for 266- and 248-nm excitation in the temp. range of 300-650 K in a nitrogen/oxygen bath gas of 1 bar total pressure with oxygen partial pressure in the range 0-400 mbar. Contrary to a popular assumption, the toluene LIF signal is not directly proportional to the fuel-air ratio (termed the FAR-LIF assumption) for many conditions relevant to reciprocating IC engines. With increasing temp., a higher oxygen partial pressure is required to justify the FAR-LIF assumption. The required oxygen pressure becomes unrealistic (>5 bar) for T>670 K at 266-nm excitation and for T>625 K at 248-nm excitation.