Chemically defined binary fuel mixtures of iso-octane (baseline fuel), toluene, methyl tertiary butyl ether (MTBE), and di-isobutylene (DIB) have been run in a production spark ignition engine at various speed/load conditions, with the engine in a clean (deposit-free) and deposited state. Pre-catalyst exhaust gases were analyzed for NOx, total unburned hydrocarbons (UHC), and speciated unburned hydrocarbon concentrations.As toluene was added to the baseline fuel, NOx concentrations increased but total unburned hydrocarbons remained constant. Total unburned hydrocarbons and NOx were unaffected by MTBE. DIB reduced total unburned hydrocarbon emissions but had little effect on NOx.Pure iso-octane produced seven major unburned hydrocarbon species. All the fuels when added to iso-octane resulted in changes in the existing species as well as the production of new ones. The major hydrocarbon species observed in the exhaust were consistent with current intermediate temperature oxidation mechanisms for the fuels studied.NOx concentrations increased substantially when the test fuels were repeated in the engine after deposits were formed; however, total and speciated unburned hydrocarbons were not significantly affected.