Synergistic effects of cationic gemini 1,4-bis(3-dodecylimidazolium-1-yl) butane bromide (C-12-4-C-12)Br-2 and anionic sodium lauryl sulfate (SLS) on the interfacial composition, solubility, and solubilization capacity of the microemulsions were investigated. When the mole fractions of (C-12-4-C-12)Br-2 in a (C-12-4-C-12)Br-2 and SLS mixture (X(C12-4-C12)Br2) is in the ranges of 0.7 to 1 and 0 to 0.1, the three-phase microemulsion region appears. The solubilization capacity (SP*) of the microemulsions containing both (C-12-4-C-12)Br-2 and SLS surfactants become much greater in contrast to the microemulsions that are either (C-12-4-C-12)Br-2-based or SLS-based. The solubilization capacity (SP*) reaches a peak value at X(C12-4-C12)Br2 = 0.7. The effects of oils, NaCl contents, and temperatures, on the oil +(C-12-4-C-12)Br-2 + SLS + butan-1-ol + aqueous NaCl microemulsion system's epsilon-beta phase diagrams were also discussed.