Nonequilibrium precursor mediated kinetics has been discovered for reactions of gaseous molecules at high temperatures. A theoretical analysis was carried out on dimerization of midsize polycyclic aromatic hydrocarbons (PAH), the presumed critical step in formation of carbonaceous particles in terrestrial and extraterrestrial environments. The nonequilibrium precursor state originates from inelastic collisional dynamics of two PAH monomers, with low-frequency modes acting as a sink for translational energy in the reaction coordinate. Owing to the prolonged lifetime of the nonequilibrium physical dimer, the probability of chemical dimerization increases by an order of magnitude. This phenomenon brings us closer to a solution for the carbon-particle inception puzzle and should prove useful for the fundamental understanding of gas-phase chemical reactions involving large molecules.