An ordinary, unmodified port fuel injection spark ignition automobile engine with closed-loop air-fuel ratio control and a three-way catalyst was operated on two butanol isomers, n-butanol and iso-butanol, and their blends with gasoline at steady-state operating points covering both common and potentially problematic regimes. The engine control unit was able to maintain the air-fuel ratio while running on both butanol isomers and their blends with gasoline. Only small changes in the heat release rates, small and insignificant decrease in exhaust gas temperatures, and no excessive increase in emissions were observed. Under commanded enrichment operation, the maximum torque, air-fuel ratio and exhaust emissions were comparable among nearly all fuels tested. The exhaust gas temperatures were comparable among fuels, with a moderate increase observed in some regimes during operation with high share of n-butanol in fuel. For both n-butanol and iso-butanol, startability was significantly worsened with more than 30% of alcohol by volume in the fuel. In general, while differences among the fuels were observed, there were no significant adverse effects on maximum engine torque, air-fuel ratio, exhaust gas temperature, heat release rates, and the emissions of regulated gaseous pollutants both before and after the three-way catalyst. Engine durability and other issues were not examined.