The impact of localized π-radicals on soot formation is explored by considering their electronic structure and computing their relative concentrations in flame conditions. Electronic structure calculations reveal that the presence of localized π-radicals on rim-based pentagonal rings is due to aromaticity. We further calculated a complete mechanism for the formation and elimination of the site from hydrogen additions and abstractions. A batch reactor with flame concentrations of H• and H2 was used to determine the time-dependent concentration of localized π-radicals. Low temperatures ( < 1000 K) favored the fully saturated rim-based pentagonal ring. Soot nucleation temperatures (1000–1500 K) give way to unsaturated rim-based pentagons being favored. Localized π-radicals on rim-based pentagonal rings are found to be in higher concentration than the aryl-type σ-radical on the rim-based pentagon (mole fractions of 10−6−10−7) in below < 1500 K, consistent with recent experimental observations. Higher temperatures favor the σ-radical and the concentration of the localized π-radical on rim-based pentagons becomes negligible. A kinetic Monte Carlo treatment of multiple sites indicates that multiple localized π-radicals are possible on a single molecule. These results reveal the importance of localized π-radicals on rim-based pentagonal rings for PAH chemistry leading to formation of soot nanoparticles in flames involving aromatic rim-linked hydrocarbons (ARLH).