The phase behavior of dimeric (gemini-type) anionic surfactant (sodium 1,2-bis(N-dodecanoyl β-alanate)-N-ethane, GS) in water is similar to that of the corresponding monomeric surfactant (sodium N-dodecanoyl-N-methyl β-alanate, MS). Aqueous micellar phase (Wm), hexagonal (H1), bicontinuous cubic (V1), and lamellar (LR) phases are successively formed with increasing the surfactant concentration. SAXS data for cross-sectional area per surfactant molecule in H1 and LR phases reveal that GS molecules are more tightly packed in the aggregates than MS molecules, and this is due to the bridging of two hydrocarbon chains by the short spacer chain (-C2H4-). In the ternary water/surfactant/dodecane or m-xylene systems, the phase behavior of both surfactants is also practically similar. Upon addition of oil, the H1 phase is changed to the discontinuous micellar cubic (I1) phase in both surfactant systems. The phase transition mechanism is discussed according to the geometrical packing model. In the 3 wt % NaCl aqueous/MS or GS/cosurfactant-(hexanol or butanol)/dodecane systems, a single-phase microemulsion is formed at an equal brine/dodecane ratio over a wide range of surfactant concentration in the MS systems whereas the LR phase is mainly formed and the microemulsion is produced only in a very narrow region in the GS/hexanol system. By replacement of hexanol with butanol, wide microemulsion regions appear, even in the GS system. Hence, GS molecules in the surfactant layer are considered to be packed more tightly than MS, even in the presence of oil.