Quantum transition state theory was used to model two hydrogen atom abstraction reactions of hydroxyl radicals. Moments of inertia for the transition species were calculated from geometries obtained by ab initio or bond energy bond order methods. The transition state equation was fit by nonlinear least squares to experimental rate constants to determine parameters of the transition species. For the reaction of hydroxyl with ammonia, the geometric mean, Wb, of four transitional. vibrational frequencies was found to be 660 +/- 20 cm(-1), the activation barrier height, V-b, including zero point energy, to be 8.5 +/- 0.5 kJ, mol(-1), and the barrier thickness, at half its height, to be 65 +/- 30 pm. For the disproportionation reaction of hydroxyl radicals, the corresponding values of the first two parameters, based on the most recent experimental data, were 890 +/- 20 cm(-1) and 2.7 +/- 0.8 kJ mol(-1). Ab initio calculations on the latter reaction gave omega(b) = 746 cm(-1) at the HF/6-31G(d,p) level and V-b = 13.6 kJ mol(-1) at the MP4/6-311G(d,p) level. The curvature of the Arrhenius plots can be explained by the contributions of the low-frequency vibrations and, for the disproportionation reaction, of the low-lying electronic states.