Mass spectra of H2/Ar diffusion flames with benzene additive show that ions of m/z 79, 81, 91 and 95 occur within the flame. Deuteration experiments indicate that these ions have the formulae C6H7+, C6H9+, C7H7O+ and C6H7O+. They also appear in the mass spectra of flames with toluene and cyclohexane additives, but in the ICR mass spectra of H2O/C6H6 mixtures only C6H7+ was found. C6H7+, C6H9+ and C6H7O+ are more prominent early in the flames and their presence may be explained by assuming proton transfer from H3O+ or HCO+ to the unburnt additive or to the initial products of combustion, i.e. benzene, C6H8 and C6H6O (probably phenol), from hydrogen and hydroxyl radical addition and abstraction reactions. C7H7+ is more prominent in the middle of the flame, where C7H6 is presumably formed by reactions of hydrocarbon radicals.