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Phase behavior and interfacial tensions of the water—dodecane—pentanol—sodium octylbenzene sulfonate system have been investigated. New results are reported on both multiphase equilibriums and interfacial properties of microemulsion systems. Our data show that the occurrence of a Winsor's type III three-phase region is not necessarily related to a crossing of the interfacial tension curves. Moreover, under some conditions of salinity, three three-phase regions are observed; two of them are characterized by coexistence of two microemulsions in equilibrium with either an aqueous phase or an organic one. Variations in oil and water composition of the middle-phase microemulsions and interfacial tensions as the transition I → III → II occurs are consistent with a three-dimensional representation of the phase diagram although the system contains five components. In the threephase region one single relationship between the γmwinterfacial tensions and one variable of composition in one of the phases (e.g., the water concentration in the middle phase) is established; likewise a similar relationship is evidenced between the γmotensions and the considered variable of composition.