Flame-sampling photoionization mass spectrometry, using continuously tunable synchrotron radiation, offers important advantages for studies of flame chemistry. Mole fraction profile measurements for 24 flame species are presented for a fuel-rich low-pressure premixed laminar C 3 H 8 /O 2 /Ar flame. Near-threshold photoionization efficiency measurements provide selective detection of individual isomers and unambiguous identifications of other flame species of near-equal mass. The absolute molar composition of the allene and propyne isomers of C 3 H 4 was determined. Absolute cross-sections for photoionization of C 2 H 2 , C 2 H 4 , CH 3 OH, C 3 H 4 (propyne), CH 3 CHO, (CH 3 ) 2 CO, and C 6 H 6 were used to determine molecular beam mass discrimination factors valid for ion masses ranging from 26 to 78amu.