The C2H5. + O-2 reaction, central to ethane oxidation and thus of fundamental importance to hydrocarbon combustion chemistry, has been examined in detail via highly sophisticated electronic structure methods. The geometries, energies, and harmonic vibrational frequencies of the reactants, transition states, intermediates, and products for the reaction of the ethyl radical ((X) over tilde (2)A') with O-2 (X (3)Sigma (-)(g), a (1)Delta (g)) have been investigated using the CCSD and CCSD(T) ab initio methods with basis sets ranging in quality from double-zeta plus polarization (DZP) to triple-zeta plus double polarization with f functions (TZ2Pf). Five mechanisms (M1-M5) involving the ground-state reactants are introduced within the context of previous experimental and theoretical studies. In this work, each mechanism is systematically explored, giving the following overall 0 K activation energies with respect to ground-state reactants, E-a(0 K), at our best level of theory: (M1) direct hydrogen abstraction from the ethyl radical by O-2 to give ethylene + HO2., E-a(0 K) = +15.1 kcal mol(-1); (M2) ethylperoxy P-hydrogen transfer with O-O bond rupture to yield oxirane + (OH)-O-., E-a(0 K) = +5.3 kcal mol(-1); (M3) ethylperoxy alpha -hydrogen transfer with O-O bond rupture to yield acetaldehyde + (OH)-O-., E-a(0 K) = +11.5 kcal mol(-1); (M4) ethylperoxy P-hydrogen transfer with C-O bond rupture to yield ethylene + HO2., E-a(0 K) = +5.3 kcal mol(-1), the C-O bond rupture barrier lying 1.2 kcal mol(-1) above the O-O bond rupture barrier of M2; (M5) concerted elimination of HO2. from the ethylperoxy radical to give ethylene + HO2., E-a(0 K) = -0.9 kcal mol(-1). We show that M5 is energetically preferred and is also the only mechanism consistent with experimental observations of a negative temperature coefficient. The reverse reaction (C2H4 + HO2. -> (C2H4OOH)-C-.) has a zero-point-corrected barrier of 14.4 kcal mol(-1).