Recently, vegetable oils have received increased attention as a source of renewable fuels. However, the high viscosity of vegetable oils makes them problematic for long-term use in diesel engines. Therefore, vegetable oil reverse micelle microemulsions have been evaluated as an alternative method of reducing vegetable oil viscosity while eliminating the trans-esterification reaction and avoiding the unpurified glycerol and its environmental problems. Since extended-surfactants can form microemulsions with a high solubilization capacity and with a wide range of oils, extended-surfactant-based reverse micelle microemulsion systems were evaluated in this research. The objective of this research is to study the phase behavior of carboxylate-based extended surfactant microemulsion systems with the goal of formulating optimized systems for biofuel. It was found that carboxylate-based extended surfactants were able to form reverse micelle microemulsions without salt addition, thereby eliminating the phase separation and precipitation which had been observed with sulfate-based extended surfactants. In addition, fuel properties such as viscosity and temperature dependence were favorable and thus support the continued development of these surfactant-based fuel systems for use in diesel engines. (C) 2011 Elsevier Ltd. All rights reserved.