The enhanced oil recovery (EOR) through microemulsion flooding implies the formulation of a complex aqueous mixture containing surfactants, co-surfactants, cosolvents, or viscosity-increasing polymers, among other additives. The optimal formulation is associated with a three-phase behavior, in which the interfacial tension becomes significantly low. One parameter that greatly affects this formulation is the temperature. In this work, it has been shown that the three-phase system generated when adding trihexyl(tetradecyl)phosphonium chloride ionic liquid to a water-oil mixture remains stable in a wide range of temperatures and in the presence of salt. In contrast to conventional systems, no co-surfactant is required. This thermal stability is an interesting feature from the EOR point of view. The use of higher temperatures implies that a slightly minor quantity of ionic liquid is needed to solubilize the water and oil mixtures. Moreover, when the temperature is increased, there is an important decrease of the microemulsion-water/brine interfacial tension and a mild decrease in the case of the microemulsion-oil. The trihexyl(tetradecyl)phosphonium chloride ionic liquid was proven to be an effective surface-active agent to recover oil.