Equilibrium phase diagram of the chlorobenzene-hexafluorobenzene binary system is experimentally determined by differential scanning calorimetry and X-ray diffraction analyses. This diagram is of the double eutectic type with a 1:1 crystalline molecular complex. The optimization of the complex thermodynamic quantities (melting temperature, melting enthalpy and first order polymorphic transition temperature) is carried out from the experimental data. The positive excess enthalpy of the melt resulting from repulsive interactions between C6H5Cl and C6F6 molecules is related to the low stability of the complex lattice.