Abstract The full orientation dependence of water adsorption at temperatures between 350 and 870 K was investigated on a cylindrical Si sample using Auger electron spectroscopy and synchrotron radiation excited photoemission. Below 450 K, water adsorbs quickly on (001) and all orientations containing (001) terraces or facets as well as on defects. On (001), its sticking coefficient is high and constant up to saturation at 1 2 monolayer, suggesting a mobile precursor state. This species was identified as OH + H by Ibach et al. and Chabal and Christman. The O 2s level position and the adsorbate induced Si 2p core level shifts were analysed and it is shown that the OH is converted thermally to bridge bonded O ( T ≳ 550 K) which finally penetrates and forms SiO x ( T ≳ 750 K.). In the second, slow adsorption step, depending on orientation, different fully dissociated species are found at 350 K. On (110), atomically adsorbed oxygen is observed, whereas on (111), the Si 2p level shifts indicate penetrated oxygen which probably has a strongly disturbed bond configuration. At 350 and 670 K, the slowly adsorbed total amount is nearly identical for all orientations. This could be explained assuming the same temperature and orientation independent precursor dominating the sticking behaviour.