To better understand the impact of ambient temperature and fuel on the emissions of ultrafine particles from spark ignition light-duty vehicles, experiments were conducted to examine the impact of ethanol-gasoline mixtures, compressed natural gas (CNG), and ambient temperature, on the size distribution and number emissions of particles. Vehicles were tested on a chassis dynamometer under controlled conditions of ambient temperature spanning from 60 to 85°F. A dual fuel vehicle operating with CNG and 6% ethanol-gasoline blends (E6); and a flex fuel vehicle operating with four different ethanol-gasoline blends E6, 35% ethanol (E35), 65% ethanol (E65) and 85% ethanol (E85) was evaluated to understand fuel effects. Changes in vehicle operating temperature, which included the combustion air temperature, over the ranges of 60 to 85°F showed no clear impact on fine particle emissions. Likewise, no significant changes in particle number or particle size distributions were observed between CNG and E6 fuels. However, tests with the flex fuel vehicle, which were able to examine higher ethanol concentrations in the fuel showed large decreases in particle number emissions as ethanol levels increased to E65 and E85.