In the gas phase Fe+-catalyzed polymerization of chlorobenzene Fe(C6H4)(3)(+) (1) reacts with chlorobenzene to give (C6H4)(3)(+) (2) and Fe(C6H4)(4)(+) (3) in a ratio that depends on the chlorobenzene pressure, Increasing the chlorobenzene pressure favors formation of 2. Decreasing the pressure of chlorobenzene or increasing the pressure of collision gases Ar or C6F6 favors the formation of 3. The branching ratio for the formation of 2 and 3 changes 12-fold between the high- and low-pressure behaviors. The results suggest that the loss of Fe+ from 1 to form 2 is nor a process analogous to collision-induced dissociation but involves a more active participation from the neutral chlorobenzene. A mechanism accounting for the pressure dependence is suggested, such that 1 is formed with considerable internal energy, which strongly affects the relative rate constants in the two reaction channels. At lower pressure 1 relaxes to the ground state mainly through a radiative process during the relatively longer time between collisions, whereas at higher pressure the shorter time between collisions does not allow for significant radiative relaxation, and hence, the reactions at higher pressure depict the behavior of 1 in an excited state. Incorporating a buffer gas at the higher pressure allows 1 to relax through collisional cooling and exhibit ground-state behavior. Comparison of the experimentally observed behavior of the reaction is consistent with all the important features of the behavior predicted by the proposed mechanism and yields a value for the radiative relaxation rate constant of 1.7 +/- 0.6 s(-1).