Ethanol has a high octane rating and can be added to gasoline to produce high octane fuel blends. Understanding the octane increase with ethanol blending is of great fundamental and practical importance. Potential issues with fuel flow rate and fuel vaporization have led to questions of the accuracy of octane measurements for ethanol-gasoline blends with moderate to high ethanol content (e.g., E20-E85) using the Cooperative Fuel Research (CFR™) engine. The nonlinearity of octane ratings with volumetric ethanol content makes it difficult to assess the accuracy of such measurements.In the present study, Research Octane Number (RON) and Motor Octane Number (MON) were measured for a matrix of ethanol-gasoline blends spanning a wide range of ethanol content (E0, E10, E20, E30, E50, E75) in a set of gasoline blendstocks spanning a range of RON values (82, 88, 92, and 95). Octane ratings for neat ethanol, denatured ethanol, and hydrous ethanol were also measured. One set of measurements was conducted using a CFR™ engine equipped with manufacturer-supplied enhancements (GE Energy Waukesha XCP-OA™ digital octane panel) for digital knock measurement and precise control of temperatures and fuel flow. A second set of measurements was conducted at a separate laboratory with a CFR™ engine equipped with an adjustable-orifice fuel jet. Both approaches address fuel flow issues at high ethanol concentrations.A linear molar octane blending model was found to describe most of the nonlinearity in the RON and MON data, but measured values were still somewhat greater than predicted. Deviations from the linear model can be described by a term with 2nd-order dependence on ethanol content with a single scaling parameter (Pg). The parameter Pg can be estimated from the measured octane number of a 50:50 molar alcohol-gasoline blend and the octane numbers of the gasoline (ONg) and alcohol (ONa). The octane number of any ethanol-gasoline blend (ONb) with that blendstock can then be estimated (within 1 ON) from the molar fraction of the alcohol (xa) using the following expression: ONb = (1-xa)ONg + xaONa + Pg xa (1-xa)(ONa - ONg).This study supports a companion paper (SAE 2012-01-1277) in which a state-of-the-art single-cylinder engine equipped with multiple fuel injection systems was used to evaluate the knock-limited performance of the ethanol-gasoline blends described herein and to evaluate the relevance of octane ratings and heat of vaporization as predictors of this performance.