Interactions between didodecyldimethylammonium (DDA) bilayers were measured using osmotic stress in the presence of bromide and chloride as mixed electrolyte. DDA bromide exhibits a discontinuity in its force curve, corresponding to a coexistence region in the DDAB/water phase diagram. whereas DDA chloride has a continuously repulsive interaction profile. Mixed electrolyte systems, interacting at constant electrolyte chemical potential, also display demixing. The selective binding of bromide and chloride in lamellar phases has been measured as a function of interlamellar separation and compared with selective binding to the air/solution interface. A site-binding model was used to interpret the selective binding and osmotic stress results and suggests that these bilayers interact at constant potential.