In situ X-ray absorption spectroscopy (XAS) was applied to investigate the Sn underpotential deposition (UPD) on Ni surface from 0.2 M HClO4 solution containing 10(-3) M Sn2+ with relation to the inhibition effect of Sn on aqueous corrosion of Ni. The periodical emersion method under potentiostatic polarization, using the surface-roughened Ni plate (surface roughness S-r = 78.3) as a working electrode was employed to detect sensitively the sub-monolayer coverage of Sn on Ni. The Sn K-edge absorption spectra in a scanning XAS mode were measured by monitoring the Sn K-alpha 1 fluorescence line. The Sn K-edge absorption near-edge structure (XANES) in the Sn-UPD potential region has revealed that the Sn-UPD layer on Ni is oxygenated. The extended X-ray absorption fine structure (EXAFS) analysis was performed with a two shell fit of the nearest neighbor Sn-Ni and Sn-O interactions, assuming that the uppermost Ni surface exposed to the solution is mainly oriented to the (111) plane. The results have indicated that Sn atoms are substituted like a surface alloy at face-center-cubic sites in the first Ni layer and further bonded with oxygen atoms. The strong inhibition effect of Sn on aqueous corrosion of Ni is ascribed to the bond between Sn and O atoms in. addition to the bond between Sn and Ni atoms. (C)2014 The Electrochemical Society.