The kinetics and mechanism of the reaction between amidogen radical and hydroxyl radical have been theoretically investigated on the lowest singlet and triplet surfaces. The singlet surface consists of two long-lived chemically activated NH(2)OH* and NH(3)O* intermediates with 10 different channels. A hydrogen abstraction channel on the triplet surface proceeds through van der Waals complex in both reactant side and product side to produce NH(3) + O((3)P). The effect of multiple reflections of the van der Waals complexes on the rate constant is investigated. Multichannel RRKM-TST calculations have been carried out to calculate the individual rate constants for the formation of those products that proceed through activated adducts on the singlet surface. The rate constants for direct hydrogen abstraction reactions were calculated by using direct-dynamics canonical variational transition-state theory with small curvature approximation for tunneling.