Temperature dependence of the rate constant of the NH3 + H -> NH2 + H-2 reaction and its reverse was calculated for 200-1000 K temperature range within the quantum instanton method. The potential energy surface PES-1997 was used. Not being of the heavy-light-heavy type, the hydrogen abstraction by another hydrogen allows safe neglection of recrossing effects. Curvature of the Arrhenius plot is present at low temperatures, which is indicative of quantum effects due to light hydrogen atoms. Additionally, a fully quantum mechanical method of calculating the temperature dependence of the equilibrium constant is presented and applied. The method is based upon path-integral Monte Carlo simulations and subsequently on thermodynamic integration. (C) 2012 Elsevier B.V. All rights reserved.