The interaction between anionic and cationic surfactants was investigated by means of surface tension, conductivity, and nuclear magnetic resonance. It was found that a strong interaction exists between anionic and cationic surfactants and the mixed surfactant has a hydrophobic property. The phase diagram has been determined as a function of temperature for the waterstearyltrimethylammonium chloride (STAC)-sodium laurate (NaL). The Krafft point rose remarkably in equimolar mixtures for this system. The phase diagram has been determined as a function of temperature for the STAC-sodium-N-lauroyl-N-methyl-β-alanine (NaLMA) system. The liquid-liquid phase separation phenomenon was observed around an equimolar mixture. It can be identified with the cloud point, which is shown by nonionic surfactants. This phenomenon seemed to be caused by the decreased solute-solvent interaction (i.e., dehydration of the amido group contained in NaLMA molecule) as the temperature is raised. The decreasing effect of protein (ovalbumin) denaturation was observed in the high area of the mole fraction of cationic surfactant in the cationic-anionic system. We believe that this is due to the remarkable lowering of the monomer concentration by the formation of a hydrophobic complex