The interfacial and thermodynamic properties of water/cetyltrimethylammonium chlorine (CTAC)/alkanol/diesel oil w/o microemulsion systems were investigated by the method of dilution. Alkanols with different carbon numbers (n-butanol, n-pentanol, iso-pentanol, n-hexanol, n-octanol) were used to investigate the effect of cosurfactant on the formation of diesel oil microemulsion. The composition of the cosurfactant and the surfactant in the interfacial region, the distribution of the cosurfactant between the interfacial region and the continuous oil phase, and the energetics of transfer of the cosurfactant from the oil to the interface were estimated. (C) 2008 Elsevier Ltd. All rights reserved.