The calculated difference in the standard heat of formation Δ ΔfH°(298.15) of n- and i-C4H3• free radicals is 37.9 kJ mol−1 for G3MP2B3 and 45.0 kJ mol−1 for CCSD(T)-CBS (W1U) calculations, which seems to preclude the direct even-carbon radical pathway to benzene and higher PAH (polycyclic aromatic hydrocarbon) formation including soot in a hydrocarbon flame. For the phenyl-type σ-radicals listed in the title, absolute values of ΔfH°(298.15) have been calculated using G3MP2B3-computed values of bond dissociation energies D°(298.15) and combined with experimental values of ΔfH° (298.15) for the parent hydrocarbon because of a slight systematic overprediction of the thermodynamic stability of large PAHs by the applied computational G3MP2B3 method. Standard enthalpies of formation ΔfH°(298.15) as well as absolute entropies S° and heat capacities C°p are given for a series of π- and σ-free radicals important to combustion as a function of temperature. A spread of roughly 40 kJ mol−1 in the average CH bond strength of PAH leading to σ-radicals has been calculated, the lowest leading to 4-phenanthryl (463.6 kJ mol−1), the highest leading to 2-biphenylyl radical (502.5 kJ mol−1). © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 395–415, 2008