A vehicle test program was conducted at the Environmental Protection Agency's National Vehicle and Fuel Emissions Laboratory to provide data on the relationship between fuel properties and exhaust emissions of nonmethane hydrocarbons (NMHC), NOx, and CO. This study, Phase III, is the third in a series of programs sponsored by the Agency. This Phase III program consisted of 19 light-duty high and normal emitting vehicles tested on 10 different fuels. The properties for each test fuel were specified in order to examine seven separate fuel effects on exhaust emissions; interactions between olefins and volatility, interactions between olefins and sulfur, very high and very low levels of sulfur, low levels of aromatics, low volatility, and low levels of olefins. For all of the fuels tested, the normal emitter vehicles produced greater percentage reductions than the high emitters. The data in this work showed lower NMHC emission reduction than predicted by the complex model. The results indicate that the complex model may exaggerate impacts on NOx emissions from these fuels. No significant interactive effect between olefins and RVP or between olefins and sulfur was found.