An investigation was made to determine the effects of hydrogen enrichment of ethanol at ultra-lean operating regimes utilizing an experimental method. A 0.745 L 2-cylinder SI engine was modified to operate on both hydrogen and ethanol fuels. The study looked at part throttle, fixed RPM operation of 0%, 15%, and 30% hydrogen fuel mixtures operating in ultra-lean operating regimes. Data was collected to calculate NO and HC emissions, power, exhaust gas temperature, thermal efficiency, volumetric efficiency, brake-specific fuel consumption, and Wiebe burn fraction curves. It was shown that hydrogen enrichment of ethanol demonstrated an ability to reduce NOx and stabilize and accelerate the combustion process. The experiments showed that operating near the LOL at both 15% and 30% hydrogen by volume reduced engine out NOx emissions by more than 95% as compared to stoichiometric gasoline operation. This reduction is comparable to the efficiency of modern three-way catalyst and could offer an alternative to current NOx reduction technologies. Power, thermal efficiency, and volumetric efficiency were not affected by the hydrogen mixture at a given equivalence ratio. However, hydrogen addition allowed an increase in the lean operating limit which helped further reduce NOx emissions, but also at reduced power and thermal efficiency. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.