Reactive scattering of O(P-3) atoms with C2H5I molecules has been studied at initial translational energies E similar to 51 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. A mildly forward- and backward-peaked angular distribution of IO product observed at lower initial translational energy becomes backward-scattered at higher initial translational energy. An isotropic angular distribution is observed for HOI product at lower initial translational energy which becomes forward- and backward-peaked, slightly favoring the backward direction at higher initial translational energy. Extended phase space calculations indicate that reaction leading to IO product proceeds via intersystem crossing from the lowest triplet (3)A" potential energy surface to a bound OIC2H5 intermediate on the singlet (1)A’ potential energy surface in small impact parameter collisions at low initial translational energy, However, the emergence of backward-scattered IO product indicates the onset of direct reaction over the triplet 3A" potential energy surface at higher initial translational energy, The formation of HOI reaction product arises from the singlet OIC2H5 complex via a five-membered ring transition state where the H atom is abstracted from the terminal CH3 group at the top of a barrier leading to vibrational excitation of HOI and repulsion between the reaction products in this strongly exoergic exit valley of the potential energy surface.