Electrochemical insertion by a set of different ions (H+, Li+, Na+ and K+) into a tungsten oxide thin film was studied by photoelectron spectroscopy. The tungsten oxide thin film incorporating Si atoms was produced from a silicotungstic acid (SiWA) solution. The insertion compounds were measured by core level photoelectron spectroscopy (W 4f) and the contributions from ions of different oxidation states could be monitored simultaneously. SiWA films having a W6+/W-tot ratio of 0.7 could be prepared for all cations investigated. At this ratio the W 4f core level electronic structure for H+ inserted SiWA films was found to be very similar to that of H+ inserted into crystalline monoclinic WO3 in that both films show the presence of W4+, W5+ and W6+. The measurements on Li+ inserted SiWA films indicate an electronic structure very similar to that of the smaller (H+) ion. The K+ inserted film displays a similar behaviour although the existence of W4+ was difficult to ascertain. Interestingly, a different behaviour was observed for the Na+ inserted compound. In this case, the binding energy shift of the W 4f peak upon reduction is clearly different from that obtained for the other insertion materials. (C) 2004 Elsevier B.V. All rights reserved.