In this work, we investigate oxidative steam reforming (OSR) of ethanol on a series of metals under various catalytic conditions (H2O/ethanol and O-2/ethanol ratios) to understand the reaction mechanism and to optimize the catalytic conditions for optimal hydrogen production. There are three reaction pathways for OSR using these metals. Ethanol can be oxidized to acetaldehyde on Cu, Ag and Au, and it can be dehydrated to form ethylene on Co, Ni, Pd and Pt. Ethylene can form coke and degrade catalysts after the long-term OSR. In the third pathway, ethanol preferentially breaks its C-C bond and is further oxidized to CO or CO2 on Ru, Rh and Ir, providing optimal hydrogen production. In addition, increasing H2O/ethanol and O-2/ethanol ratios can improve catalytic activity, attributable to atomic oxygen from H2O and O-2 efficiently rupturing the C-C bond of ethanol. This concept explains the improved performance of OSR on the CeO2-modified catalyst, which shows better oxygen storage capability. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.