The toluene radical ion C6H5CH3.+, generated by resonance two-photon ionization, does not react with a single isobutene molecule (i-C4H8) which has a significantly higher ionization potential (Delta IP = 0.42 eV). However, a reaction is observed involving tyro i-C4H8 molecules, to form the dimer ion C8H16.+. A coupled reaction of dimer formation and charge transfer to the dimer is exothermic if the product is an ionized hexene with a low IP. Correspondingly, the observed nominal second-order rate coefficients, (5-25) x 10(-12) cm(3) s(-1), are enhanced by a factor of > 10(5) over the expected value for direct endothermic charge transfer. Pressure and concentration effects suggest a sequential mechanism that proceeds through a C6H5CH3.+(i-C4H8) reactive pi complex. The complex can isomerize to a nonreactive CH3C6H4-t-C4H9.+ adduct, or react with a second i-C4H8 molecule to form a C6H5CH3.+ (i-C4H8)(2) complex, in which the olefin molecules are activated by the aromatic ion. Similar reactions are observed in the benzene/propene system with a somewhat larger Delta IP of 0.48 eV, suggesting that the charge density on the olefin in the complex is still sufficient to activate it for nucleophilic attack. However, aromatic/olefin systems with Delta IP > 0.87 eV show no olefin dimer formation, At low [i-C4H8] and [Ar] number densities, the rate of formation of C8H16.+ is proportional to [i-C4H8](2)[Ar]. The corresponding fourth-order rate coefficient shows a strong negative temperature coefficient with k = 11 x 10(-42) cm(9) s(-1) at 300 K and 2 x 10(-42) cm(9) s(-1) at 346 K, suggesting that the mechanism fan be efficient in low-temperature industrial and interstellar environments. The direct formation of the dimer bypasses the monomer olefin cation and its consequent side-reactions, and directs the products selectively into radical ion polymerization. The products and energy relationships that apply in the gas phase are observed also in clusters.