The interfacial and thermodynamic properties of aqueous HCl + cetyltrimethylammonium chlorine (CTAC) + alkanol + diesel oil w/o microemulsion systems were investigated by the method of dilution. Aqueous hydrochloric acid solution (mass fraction 20 %) was used to investigate the effect of HCl on the formation of 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, the energetics of transfer of the cosurfactant from the oil to the interface, and the structural parameters of the microemulsion were estimated at (303, 313, and 323) K.