Molecular orbital theory was used to study the stationary points on the reaction-path of the hydrogen abstraction reaction between ammonia and an oxygen atom. In the C-s symmetry, this reaction proceeds over two potential energy surfaces, (3)A' and (3)A ", With a hydrogen-bonded complex in the exit channel. From the analysis of the reaction-path curvature, we find qualitatively that excitation of the NH3 stretch and bend modes might enhance the forward rate constants and that the OH stretch and NH2 bend modes of the products could appear vibrationally excited, although, as a result of the randomization of the energy favored by the deep hydrogen-bonded well, these modes will appear less vibrationally excited than those in other direct reactions. The total forward thermal rate constants we:re calculated from the sum of the calculated rate constants for the two surfaces using variational transition-state theory with semiclassical transmission coefficients over a wide temperature range, 500-2000 It. The calculated rate constants show reasonable agreement with-experimental data, with a more pronounced curvature in the Arrhenius plot than in the experimental data.