In this paper, an onboard reformer and a dual-fuel (hydrous-ethanol and gasoline) supply system were designed to examine experimentally the reforming performance of hydrous-ethanol for an on-line, operating engine, and a series of optimization and comparison experiments were conducted. The results show that HE75 (75% hydrous-ethanol, i.e., ethanol with 25% water volume content) conversion first increases and later decreases with the temperature and reaches its peak at a temperature of approximately 675 K. The effects of the flow rate and temperature on the product distribution are minimal. Compared to the prototype gasoline-fueled engine, the average decreases of the equivalent specific fuel consumption, NOx emissions, CO emission and total hydrocarbon emissions for the optimized engine fueled with hydrogen-rich reformates are 6%, 70%, 50% and 80%, respectively. This preliminary experiment suggests that the utilization of hydrous, rather than anhydrous, ethanol in a spark ignition engine by the onboard steam reforming of ethanol may represent a sustainable alternative energy source. Copyright. (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.