The water adsorption on the bare and H-terminated Si(1 0 0) surfaces has been studied by the BML-IRRAS technique. It is found that H-terminated surfaces are much less reactive compared to the bare silicon surfaces. The (1×1)-H and (3×1)-H surfaces show similar and less reactivity pattern compared to the (2×1)-H surface. At higher exposures, the water reaction with coupled monohydride species provides an effective channel for oxygen insertion into the back bonds of dihydride species. It is not attributed to the H-Si-Si-H + H 2O → H-S-Si-OH + H2, which could give rise to the characteristic Si-H and Si-OH modes, respectively at 2081 and 921 cm -1. A more suitable reaction mechanism involving a metastable species, H-Si-Si-H + H2O → H2Si ⋯ HO-Si-H (metastable) explains well the bending modes of oxygen inserted silicon dihydride species which are observed relatively strongly in the reaction of water with H-terminated Si(1 0 0) surfaces. © 2004 Elsevier B.V. All rights reserved.