The stability diagrams of ethanol (o),biodiesel (b)-diesel blends, wore studied at different temperatures, It was found that biodiesel acts as a stabilizer component in P-diesel blends, except at low temperatures, where it favors the formation of a gelatinous phase, Three blends (two c-diesel and all e b-diesel) were selected to be tested in a diesel engine, and their performance and emissions were compared to those of a reference diesel fuel. The results show that, with increasing ethanol content in the blends, hydrocarbon emissions increase significantly because of the high heat of vaporization of alcohol, thus promoting the appearance of a nuclei mode. With all of the blends tested, reductions in smoke opacity and particulate matter emission,, with respect to (diesel fuel are obtained, but these decrease were not lineal with the oxygen content, The oxygen provided by ethanol resulted in more efficiency In the opacity reduction than the oxygen provided by methyl ester (c-b-diesel blend), but in the case of particulate matter emissions, the opposite trend was observed.