Fourier transform infrared spectroscopy has been used to study thermotropic phase transitions of a 20 g% water-octadecyltrimethylammonium chloride (ODAC) system. In this system, there appear three successive phases of coagel, gel, and liquid crystal with increasing temperature. In the coagel phase, the methylene chains take the trans-zigzag conformation packed in parallel with each other. The hydrophilic part of ODAC is in a fixed state, and the presence of bound water is evident. In the gel phase, the rotational motion of the methylene chain around the chain axis occurs in the hexagonal lattice. The hydrophilic part is in a fused state. The water spectrum becomes similar to that of liquid water. In the liquid crystalline phase, both hydrophobic and hydrophilic parts of ODAC are in fused states. The water spectrum also resembles that of liquid water.