The phase diagram of the binary system CTAB (cetyltrimethylammonium bromide): OIPh (o-iodophenol) was investigated by DSC and TG methods along with polarizing-microscopic and visual observations. It was found that CTAB and OIPh produced a 1 : 3 intermolecular complex in addition to the 1 : 1 complex already reported. Both complexes underwent incongruent melting at about 305 K and 365 K, respectively. Another complex of 2 : 1 ratio was observed in the high temperature region, which formed a solid solution of mesomorphic nature with the high-temperature metastable phase of CTAB over Limited composition range. The thermodynamics of formation of the 1 : 1 complex in the crystalline state is discussed.