Pure ethanol is being used as a spark-ignition engine fuel in Brazil, and is being considered for use in the U.S. and elsewhere. Efficiency and exhaust emissions with ethanol were quantified using a single-cylinder engine at compression ratios from 7.5 to 18, at equivalence ratios from 1.2 (rich) to the lean limit, and at MBT spark timing. Results were compared to those with gasoline at 7.5 compression ratio.With ethanol, compared to gasoline at the same compression ratio, engine thermal efficiency increased 3 percent, peak nitrogen oxide emissions decreased 40 percent, unburned fuel and carbon monoxide emissions were similar, and aldehyde emissions increased 110-360 percent. Increasing compression ratio from 7.5 to 18 with ethanol increased efficiency 18 percent, peak nitrogen oxide emissions 30 percent, unburned fuel emissions 25-200 percent, and aldehyde emissions 50-140 percent. Regression analysis indicated that the increased aldehyde emissions at high CR's may result from reduced exhaust temperatures. As exhaust temperature decreases, oxidation of aldehydes in the exhaust system decreases.