The sodium salt of N,N′-hexane-bis (1-dodecen-1-ylsuccinamic acid) is an anionic dimeric (gemini) surfactant. A flooding penetration scan of this surfactant in water demonstrates a sequence of lyotropic phases at room temperature (20 °C). Preparation of surfactant–water mixtures has resulted in a phase diagram which shows that the same sequence of phases exists up to 100 °C. These phases are tentatively assigned to the sequence: micellar to normal hexagonal (H1) to cubic (V1) to lamellar (Lα). The interfacial tension at the n-heptane/water interface has been determined in the presence of this surfactant. The surfactant head group area at the interface is large (2.8±0.3 nm2 at 298 K) and the interfacial tension above the critical micelle concentration is low (7 mN m−1), but considerably higher than the ultra-low values that have been reported for cationic dimeric surfactants at various hydrocarbon–water interfaces.