The effects of ethanol fumigation on the performance and emissions of a four-cylinder, turbocharged diesel engine have been investigated. The effects of speed, load, alcohol proof, and the fraction of the engine's power supplied by the alcohol are presented. Comparisons are made with methanol and water injection. Analysis of the results shows that methanol and ethanol have almost identical effects when compared on an equal energy basis that includes the enthalpy of vaporization of the alcohol and water. The indicated thermal efficiencies of the alcohol and diesel fuel are separated, showing that the alcohol utilization is not affected by proof or fraction of power contributed by alcohol. A dramatic reduction in NOx emission suggests that fumigation may have potential as an emission control technique in diesel engines. A stoichiometric, adiabatic flame temperature is calculated and used to determine the contribution of lower combustion temperature to the decrease in NOx emission. A reduction in NOx emission beyond that which can be explained by a flame temperature reduction is found and attributed to changes in the mode of combustion.