Isomers of the C 5 H 3 and C 5 H 5 free radicals formed in benzene/oxygen flame are identified by analyzing the observed photoionization efficiency spectra with the aid of theoretical simulations based on calculated ionization energies (IEs) and Franck–Condon factors. For C 5 H 3 , the results reveal that two linear isomers (i-C 5 H 3 and n-C 5 H 3 ) are formed in the benzene flame, while, for C 5 H 5 , both linear and cyclic isomers (l-C 5 H 5 and c-C 5 H 5 ) are formed. Adiabatic IEs for i-C 5 H 3 , n-C 5 H 3 , l-C 5 H 5 and c-C 5 H 5 are measured to be 8.21, 8.32, 7.83 and 8.44eV, respectively (all with an uncertainty of ±0.05eV).