The interactions in mixtures of anionic/cationic surfactants were investigated by several methods (conductivity, surface tension, turbidity, electrophoretic mobility, and EMF measurements). The mixtures studied were sodium tetradecylsulfate/tetradecyl pyridinium chloride and sodium dodecyl sulfate/dodecylammonium chloride. At least three different concentration regions can be differentiated in the region of mixed micelles depending on changes in the surfactant molar ratio. A model for their interface between mixed micelles and the surrounding electrolyte is supposed to incorporate the headgroups of both surfactants and pairs of headgroups-counterions of the surfactant in excess.