This paper examines the effect of charge cooling on the Research Octane Number (RON) of ethanol/gasoline blends. While gasoline is fully vaporized prior to entry into the engine in a standard RON test, significant charge cooling is observed for blends with high ethanol content, with the presence of a near-saturated and potentially two-phase air-fuel mixture during induction. Thus, the relative significance of the charge cooling and the autoignition chemistry cannot be determined from the standard RON test.In order to better delineate the effects of charge cooling and autoignition chemistry, a so-called ‘modified RON’ test is therefore devised in which the temperature of the air-fuel mixture entering the engine is fixed and representative of that observed for primary reference fuels (PRFs). Thermodynamic modeling of this modified RON test suggests the mixture is always fully vaporized prior to compression, and that the modified RON test maintains much more constant temperatures during compression, regardless of the enthalpy of vaporization. The modified RON test thus appears to be a more practical means of examining the significance of autoignition chemistry of various fuels, particularly gasoline blends with significant alcohol content.