Bioethanol is an attractive fuel due to its renewable origin and its oxygen content, but it is unable to be used directly in diesel engines. Although biodiesel can be produced with bioethanol through ethanolisis, direct blending of ethanol and diesel fuel, called e-diesel, has at least the same potential to reduce particulate emissions, despite their much lower production cost. The main drawback is that ethanol is immiscible with diesel fuel over a wide range of temperatures, leading to phase separation. Consequently, in many cases the presence of a surfactant and cosolvent additive in the e-diesel blend becomes necessary. In this paper the conditions in which the e-diesel blends are stable have been studied. The stability of samples is affected by three factors mainly: temperature, water content and initial ethanol content. The results show that the presence of water in the blends, low temperatures and high ethanol contents favour the phase separation whereas the presence of the additive leads to the opposite effect. These effects have been quantified through level curve maps for stability and for separation time. (c) 2006 Elsevier Ltd. All rights reserved.