Freezing and melting behaviors of concentrated cationic surfactant-water binary systems were investigated by using differential scanning calorimetry and polarized light microscopy, and the phase diagrams around the freezing temperature of water were constructed. The surfactants studied were decyl-and octyl-trimethylammonium bromide (abbreviated as DeTAB and OTAB). The OTAB-water system forms a lyotropic-liquid crystal in highly concentrated region of ca. 80 wt% OTAB. The head groups of OTAB strongly interact with water molecules and reduce freezing point of water down to -33 degrees C. To clarify the structure of the liquid-crystalline phase, small angle X-ray diffraction study was carried out on DeTAB-water system as a function of composition at various temperatures. The observed diffraction patterns indicate the formation of the hexagonal phase, H-alpha, and suggests that the H-alpha phase consists of finite cylinder micelles with length increasing with surfactant content.