The effect of gasoline/ethanol blends in different blending proportions (E0, E10, E20, E50, E70, E85 as #% by volume) on the characteristics of size-resolved particulate number and mass concentrations was investigated in a single-cylinder optical access engine using a differential mobility spectrometer (DMS500) under cold and warm conditions (20 degrees C, 80 degrees C) at a stoichiometric condition (1500 rpm, 0.5 bar manifold absolute pressure). The effects on the fuel spray characteristics were also investigated by capturing the injection images and by post-processing to compare the temporal development of the spray with the different fuels. In-cylinder fFID (fast Flame Ionisation Detector) measurements were undertaken to assess the mixture in-homogeneity in the region of the spark plug. As the ethanol volumetric percentage increases, both the total Pn (Particulate number) and Pm (Particulate mass) increase by a maximum of 16 and 11 for cold conditions and 7 and 8 for warm conditions. This is in agreement with the natural flame chemiluminescence images which showed that fuels with high ethanol proportions generated more soot than fuels with small blending percentages of ethanol. Spray images and combustion analysis showed that ethanol addition results in protracted injection and combustion periods. The effect on spray characteristics is more profound under cold conditions than under warm conditions. The mixture in-homogeneity increases as the ethanol content increases for E50, E70, and E85 and this would explain the increased level of particulate emissions, although the trend is less clear for the lower ethanol content fuels. (C) 2012 Elsevier Ltd. All rights reserved.