A low-pressure premixed toluene/O-2/Ar flame with the equivalence ratio of 1.90 was investigated using tunable synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry. Combustion intermediates up to C19H12 were identified by the measurements of the photoionization mass spectrum and photoionization efficiency spectrum. Mole fraction profiles of flame species were evaluated from the scan of burner position at photon energies near ionization thresholds. Furthermore, flame temperature was recorded by a Pt/Pt-13%Rh thermocouple. The comprehensive experimental data concerning the flame structure facilitate the discussion about the flame chemistry of toluene and other monocyclic aromatic fuels. Benzyl and benzene were found to be major primary intermediates of toluene degradation; and benzene is suggested to originate mainly from fuel degradation instead of radical recombination channels in fuel-rich monocyclic aromatic hydrocarbon flames. On the basis of the intermediate identification, comparison is made among the current mechanisms relevant to the formation of polycyclic aromatic hydrocarbons (PAHs). It is concluded that the molecular growth process in this flame is consistent with the synergy of the hydrogen-abstraction-carbon-addition (HACA) mechanism and the resonantly stabilized radical addition mechanism. In particular, the HACA mechanism can connect a great deal of aromatic intermediates observed in the present work and consequently explain the regular ring enlargement by consecutive addition of 2 or 4 carbon atoms, while the resonantly stabilized radical addition mechanism may have marked and sometimes predominant influences on the formation of many typical PAHs.