This paper reports the Research (RON) and Motor (MON) Octane Numbers of ethanol blended with production gasoline, four gasoline surrogates, n-heptane, isooctane and toluene. The ethanol concentration was varied from zero to 100%, resulting in a clear picture of the variations of the RONs and MONs in all cases. Of initial interest are the RONs and MONs of ethanol blended with an Australian production gasoline and with several US production gasolines. The observed differences then prompt a systematic study of the variation in the RONs and MONs of ethanol blended with four gasoline surrogates, as well as with n-heptane, isooctane and toluene. Both n-heptane, isooctane and their Primary Reference Fuels (PRFs) are shown to blend synergistically with ethanol, whilst toluene blends antagonistically. Consistent with these trends, a progressive increase in the toluene content in Toluene Reference Fuels (TRFs) of a constant RON results in increasingly linear ethanol/TRF blending. Together, these results show that the antagonism of ethanol's blending with toluene acts against its synergism with isooctane and n-heptane, and more broadly suggest that the antagonism of ethanol's blending with aromatics may act against its synergism with paraffins. If correct, this explains trends observed both in the literature and in this study, and has implications for fuel design. (C) 2013 Elsevier Ltd. All rights reserved.