The contribution of radical sites as active sites in several carbon addition and oxidation reactions under combustion conditions is assessed by comparing predicted rates of reactions involving radical sites against experimental measurements. The types and strength of C−H bonds in the peripheries of PAH molecules and in the surfaces of carbonaceous solids are considered, and radical-site concentrations are computed assuming equilibration of hydrogen abstraction by H atom and hydrogen addition by H 2 . Computed populations of radical sites are consistent with observed trends in radical/parent concentration ratios measured in flames. Calculated extents of radical formation in PAH molecules and in the surfaces of carbonaceous solids in combustion are substantial, e.g., ≈0.1 to 0.3 radical sites per original C−H bond. For large PAH of interest as soot precursors, the estimated average number of radical sites per molecule is ≈5 to 7 with the distribution extending up to 10 or more. Rate coefficients based on the computed radical-site populations are comparable to experimentally determined rate coefficients in several cases including soot oxidation by O 2 and soot growth by C 2 H 2 , assuming both of these reactions are additions involving radical sites in the soot, and soot and tar oxidation by OH, soot mass growth by tar addition, and reactive coagulation of soot precursors, assuming these reactions are radical recombinations. Calculated radical-site populations and measured concentrations of radicals in flames indicate that recombination of CH 3 , C 3 H 3 and other small radicals with radical sites in tar and soot would contribute significantly to tar and soot formation if the recombination collision efficiencies are 0.1 to 1.