W 4f and O 1s X-ray photoelectron spectra for silicotungstic acid, H4SiW12O40, in pure and silica-supported form are reported. W 4f XP spectra for the supported acid are analysed in terms of contributions from two W(VI) spin-orbit doublets arising from tungsten atoms in terminal W=O bonds some of which directly interact with the silica surface. At low loading (3.2 wt.%) significant changes in the relative contributions and binding energies of the two spin-orbit doublets are taken as evidence of a strong interaction of individual [SiW12O40](4-) anions with highly active sites on the silica surface. It is suggested that selective ordering of silanol groups can occur on the silica surface in order to accommodate the adsorption of individual [SiW12O40](4-) anions. (C) 2008 Elsevier B.V. All rights reserved.