The auto-ignition, stability and duration of the combustion process in an HCCI engine, depend on the interplay between the in-cylinder thermal environment and a given fuel's chemical kinetics. Fuel available in the US market differs appreciably in composition and auto-ignition chemistry, hence strategies intended to bring HCCI to mass production must account for this fuel variability. To address this issue, a test matrix of eight gasoline fuels, comprised of blends made from refinery streams, was created, and these eight fuels were run in an experimental, single cylinder HCCI engine. The properties of these fuels vary according to their volumetric concentration of ethanol, sensitivity (S = RON-MON) and volumetric content of aromatics and olefins. The fuels all have similar RON ratings, and were designed, in large part, to mimic commercial pump gasoline available in the United States. The ethanol content was increased to 20% by volume in some fuels, specifically to investigate the impact of the oxygenate fraction on HCCI. This concentration is higher than what is currently available on average at the pump, but is relevant in the context of recent discussions about raising the ethanol content of gasoline, in the near future. For each fuel, several types of experiments were conducted to assess the effects of increased ethanol content on HCCI combustion. Since it is not practical to investigate the ignition delay, the first set of experiments was a compensated load sweep, in which combustion phasing was matched across a sweep of fuelling rate by adjusting the intake air temperature. Next, the limits of HCCI operability across the speed range were established, by varying load until either the misfire or ringing limit was detected. Finally, air temperature sweeps were carried out to correlate fuels' properties and the sensitivity of HCCI with the in-cylinder thermal environment. Increased ethanol content led to a number of important changes in engine performance, including, but not limited to, specific fuel consumption, HCCI operating range and variation of combustion phasing over a range of intake charge temperature. (C) 2014 Elsevier Ltd. All rights reserved.