An electrochemical quartz crystal microbalance study has been,performed in order to understand the alloy electrodeposition process and especially the mechanism of oxygen incorporation during Fe-Pt electrodeposition. It reveals that the presence of Pt leads to an enhanced hydrogen evolution reaction. In comparison to the single metal depositions, an additional current and an anomalous mass increase are observed when depositing from the complete electrolyte. The corresponding films exhibit a high O content and a needle-like morphology. These characteristics are altered by nitrogen gas stirring. Fe hydroxide formation due to a pH rise at the cathode and underpotential deposition of Fe deposition due to Fe-Pt alloy formation are discussed to interpret the results. (c) 2006 Elsevier Ltd. All rights reserved.