Experimental measurements have been made of the enthalpies of mixing of isooctane and ethanol blends so as to calculate the enthalpies of vaporization of these mixtures. The enthalpy of vaporization is very important for the performance of spark ignition engines, especially those that use gasoline direct injection (GDI). High enthalpies of vaporization increase the charge cooling effect so that the volumetric efficiency is improved (thereby increasing the specific output) and a higher compression ratio can also be used, because there is a lower temperature at the start of compression. The higher compression ratio increases both the efficiency and the specific output. However, measurements reported here show that the increased enthalpy of vaporization has an adverse effect on the particulate matter (PM) emissions from a GDI engine. This is attributed to the air fuel mixture being less homogeneous.