The solubilities of benzylalkyldimethylammonium salts (BADAX) in benzene were characterized by the specific Krafft boundaries, depending on their molecular structures. BADAX affords not only the Krafft boundary but also the immiscibility gap in water or aqueous electrolyte solutions. BADAX formed aggregates in aqueous solution or in benzene when its concentration was above the critical solubility concentration (csc) of the Krafft boundary. It is assumed that the aggregate structure varies successively with the change in electrolyte concentration or oil-water ratio, The phase behavior of the mixture of BADAX, benzene, water, and electrolyte was examined as a function of temperature or a function of component composition. The conditions under which a microemulsion phase appears or coexists with other phases are clarified, and the change in aggregate structure is discussed, on the basis of the solubility and immiscibility gap.