Whereas the use of ethanol in gasoline is widely practiced as a means to reduce vehicle exhaust emissions, similar use of ethanol in diesel is not widely accepted. Barriers to the use of ethanol in diesel fuel include limited miscibility at lower temperatures and need for minor variations in fuel delivery systems to account for the different physical properties of ethanol as compared to diesel. An understanding of the phase behavior is necessary to achieve benefits and avoid problems when ethanol is used in diesel. The miscibilities of ethanol in eight diesel fuels having a wide variation in compositions were evaluated. Aromatic contents and intermediate distillate temperatures had a significant impact on miscibility limits. An emphasis was placed on the transition in phase behavior from gasoline, where phase behavior problems are consistently avoided, to US-ED diesel fuel, where phase behavior is considered a barrier to implementation. An empirical correlation was developed for predicting upper critical solution temperatures.