The focus of this study was the characterization and comparison of power-specific exhaust emission rates from a closed-loop small spark-ignited engine fuelled with ethanol and isobutanol gasoline blends.A 4-cycle Kohler ECH-630 engine certified to the Phase 3 emissions standards was operated over the G2 test cycle, a six-mode steady-state test cycle, in its original configuration. This engine was equipped with electronic ignition, electronic fuel injection and an oxygen sensor.Certification gasoline fuel was splash-blended by percent volume with ethanol and isobutanol to result in the test blend levels of E10, E15, iB16 and iB8-E10.Reductions in emission rates of carbon monoxide (up to 12.0% with the ethanol blends and up to 11.4% with the isobutanol blends) were achieved along with a reduction in total hydrocarbons (up to 10.9% with the ethanol blends and up to 8.2% with the isobutanol blends). Nitrogen oxide emissions were decreased by up to 9.8% with the ethanol blends.Emission rates of carbon dioxide were decreased by up to 3.2% with the ethanol blends and up to 4.2% with the isobutanol blends. Brake-specific fuel consumption was increased with E15 and iB8-E10. Increases in thermal efficiency, from 10.4% to 13.3% were noted with all alcohol fuel blends.The use of E10, E15, and iB8-E10 significantly increased emissions of acetaldehyde by up to 3 times that of gasoline not containing ethanol. The E15 fuel blend decreased emissions of benzaldehyde. The iB16 and iB8-E10 fuels increased emissions of propionaldehyde, and butyraldehyde/2-methylpropanal and methacrolein.