A fuel-lean laminar premixed methylmethacrylate/oxygen/argon flame at 2.67 kPa with an equivalence ratio (phi) of 0.75 has been investigated with the tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular beam sampling mass spectrometry techniques. Isomers of most observed species in the flame have been identified by measurements of photoionization mass spectra and the near-threshold photoionization efficiency spectra. Mole fraction profiles for about 42 flame species are displayed. Free radicals such as CH3, C2H3, C2H5, C3H3, C3H5, C2H5O, C4H7, C3H5O, C3H7O, C4H3O, C4H9O, C4H5O2, C4H7O2, and C5H7O2, which should be of importance in understanding the formation mechanism of some toxic substances, were detected in the flame. Moreover, no isomers of any PAHs have been detected in the lean flame. Combined with the mole fraction profiles, the formation mechanisms of the free radicals, oxygenated compounds, and other molecular intermediates are proposed and will provide important information on modeling the combustion kinetics of methylmethacrylate (MMA).