The reported oxidation reaction observed by BML-IRRAS spectra on the silicon surface system, 2H + H2O/Si(1 0 0), has been studied by an ab initio molecular orbital method. The highest transition state is found at approximate to+25 kJ/mol from the reactant energy level, and the oxidation occurs easily under the experimental condition. The present study also accounts for the reactivity deduced from the absorption bands in the IR spectra. It is noted that the quenching of the reaction by thermal relaxation is impossible because the surface is not trapped into the metastable states located much lower in energy than the reactant. (c) 2005 Elsevier B.V. All rights reserved.