Reaction mechanisms of the MgH + H -> Mg + H-2 reaction have been investigated using quantum reactive scattering methods on a global ab initio potential energy surface. There exist two microscopic mechanisms in the dynamics of this reaction. One is a direct hydrogen abstraction reaction and the other proceeds via initial formation of a HMgH complex in the deep potential well. The result of the present quantum dynamics calculations suggests that the HMgH complex formed in the reaction mainly decays into the Mg + H-2 channel via a 'roaming mechanism' without going through the saddle point region. (C) 2011 Elsevier B. V. All rights reserved.