The reaction Cl+ isobutene (i-C4 H8) was reported by Suits et al. to proceed via, in addition to abstraction, an addition-elimination path following a roaming excursion of Cl; a near-zero translational energy release and an isotropic angular distribution observed at a small collision energy characterized this mechanism. We employed a new experimental method to further characterize this roaming mechanism through observation of the internal distribution of HCl (v, J) and their temporal behavior upon irradiation of a mixture of Cl2 C2 O2 and i-C4 H8 in He or Ar buffer gas. With 1-3 Torr buffer gas added to approach the condition of small collision energy, the intensities of emission of HCl (v = 1,2) and the HCl production rates increased significantly; Ar shows a more significant effect than He because Ar quenches Cl more efficiently to reduce the collisional energy and facilitate the roaming path. According to kinetic modeling, the rate of addition-elimination (roaming) increased from k E ≈2 × 105s-1 when little buffer gas was present to ∼1.9 × 106s-1 when2-3 Torr of Ar was added, and the branching ratio for formation of [HCl (v =2)]/[HCl (v = 1)] increased from 0.02 ± 0.01 for abstraction to 0.06 ± 0.01 for roaming.