Phase equilibria data for complex systems (more than three components or two phases, high pressures, complex fluids...) are needed to develop thermodynamic models capable of correlating and predicting their behaviour. Along these lines, systems composed of water, oil and surfactant exhibit an interesting conduct. In this work, the (liquid + liquid) equilibria of a ternary system comprising water, n-dodecane and a surfactant ionic liquid, trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl) imide, have been determined at temperatures (298.15 or 348.15) K and atmospheric pressure. Winsor type III systems have been found without the need of adding any co-surfactant. The stability of the triphasic system with temperature has been tested. Some physical properties (density, viscosity and interfacial tension) for phases involved have been also determined. As expected, a minor surfactant character has been found for this ionic liquid rather than for the homologous with the chloride anion. (C) 2014 Elsevier Ltd. All rights reserved.