The microemulsion phase in the didodecyldimethylammonium sulfate (DDAS)/hydrocarbon/water system is investigated. This phase extends from the water cornrr in the ternary phase diagram. The extension of the phase for various oils is determined, and it is found that the phase is not formed for straight chain hydrocarbons having less than 10 carbons. Neither is it formed with benzene or toluene as oils. At higher oil contents the phase is in equilibrium with almost pure oil, and the boundary to this two-phase region is a straight line which connects the water apex and the oil—surfactant line. It is argued that this line is an emulsification failure boundary. As a consequence, the micellar aggregates are suggested to be spherical in shape and constant in size along this boundary. NMR diffusion studies along this line support this notion. At higher surfactant/oil ratios in the microemulsion the structure is different and there is a growth in the micellar size upon increasing the oil—surfactant concentration. It is pointed out that all the aggregates formed in the DDAS/hydrocarbon/water system have aggregate curvature toward the oil medium. This is contrary to what is observed in the corresponding systems with bromine as counterion. Possible reasons for this difference are discussed.