The surface oxidation reaction, H2O + Si(100)-(2 x 1), has been studied by an ab initio molecular orbital method with the approximation of the cluster model. The rate-determining process is ascribed to the OH migration from the metastable state, and the temperature dependence of the rates of Oatom insertion reactions forming back bonding and top layer is estimated from the barrier height of each transition state. This reaction is found to occur much faster than the quenching by thermal relaxation and the arrival to the equilibrium condition. (c) 2006 Elsevier B.V. All rights reserved.