Reactive scattering of O(P-3) atoms with C3H5I molecules has been studied at initial translational energies E similar to 46 and 16 kJ mol(-1) using a supersonic beam of O atoms seeded in He and Ne buffer gas generated from a microwave discharge source. Strongly backward peaked angular distributions of IO product scattering are observed at both initial translational energies with the peaking becoming sharper at higher energy. The product translational energy distributions are pitched at higher energy for the backward scattering and shift progressively toward lower energy as the IO scattering moves round into the forward direction. Backward scattering with high product translational energy is attributed to direct reaction in small impact parameter collisions b less than or equal to 2.5 Angstrom over the triplet potential energy surface. Scattering with low product translational energy which becomes perceptible in the forward direction is attributed to intersystem crossing to the underlying singlet potential energy surface forming a bound OIC3H5 intermediate complex. Direct reaction over the (3)A" potential energy surface plays a more significant role for the exoergic allyl iodide reaction than in the nearly thermoneutral reactions of alkyl iodide molecules, where intersystem crossing to the (1)A’ potential energy surface is the predominant reaction mechanism. The dynamical basis for this difference in mechanism is discussed in terms of the topography of the potential energy surfaces involved.