Unsupported Pt/Sn nanoparticles, that had been prepared via polyol method, were modified with ultrathin (monolayer-type) films of Keggin heteropolyacids of molybdenum and tungsten. The resulting nanostructured catalysts were investigated using both electrochemical and spectroscopic (XPS) methods. Modification of unsupported Pt/Sn with phosphotungstates or phosphomolybdates led to the improvement of the system's electrocatalytic activity in terms of relatively higher currents observed during oxidation of ethanol, especially when they were recorded and compared at such low potentials as 0.2 or 0.3 V vs. RHE. It can be concluded from the XPS spectroscopic analysis that, following modification with polyoxometallate, the oxidation state of tin has changed. An attempt was also made to distinguish and correlate distinct oxidation states of tin with the system's overall electrocatalytic activity and to explain the observed enhancement effect during ethanol electrooxidation. (C) 2011 Published by Elsevier B.V.