A detailed molecular understanding of hydrocarbon combustion processes is recognized as an important goal. The specific system studied (via ab initio theoretical methods) in this work is the reaction between the ethyl radical (CH3CH2*) and molecular oxygen (O-2) Theoretical methods used include restricted open-shell Hartree-Fock (ROHF), configuration interaction including single and double excitations (CISD), coupled cluster including single and double excitations (CCSD), and a perturbational treatment of connected triple excitations [CCSD(T)]. Basis sets up to double-zeta plus polarization (DZP) quality were used. Theoretical results indicate that there are two distinct but energetically proximate intramolecular hydrogen-transfer transition states on the potential energy surface. One of the structures corresponds to a (2)A" electronic state (C-s symmetry), the other to a (2)A electronic state (no symmetry); this latter structure is derived from a (2)A’ planar structure at which the energy Hessian possesses two imaginary vibrational frequencies. At the best level of theory used, DZP CCSD(T) including zero-point vibrational energy corrections, the (2)A" transition state lies 4.5 kcal mol(-1) above the reactants (CH3CH2 + O-2)