The singlet and tripler potential energy surfaces involved in the gas phase reactive collisions of O(P-3) and C2H5I have been studied with ab initio electronic structure computations. The collisions produce both spin-forbidden HOI+C2H4 and spin-allowed OI+C2H5 products. The calculations indicate that HOI is formed via a triplet complex and through a tripler/singlet intersystem crossing, followed by passage through a singlet intermediate and transition state for the intramolecular abstraction of beta-hydrogen. All the relevant structures for this pathway are lower in energy than the reactants, and this pathway is accessible even at low impact energies. The calculations also indicate that OI may be formed by two channels. One is the same to the above singlet pathway up to the singlet intermediate, which now dissociates endothermically without barrier to give the products. The second channel is the direct dissociation of the triplet intermediate, and is open only when an enough excess energy to surmount a triplet transition state is provided. The product energy distribution is also discussed based on the structures of transition states.