The phase diagram of the two-component system between two halomethanes, carbon tetrachloride (CCl4) and bromotrichloromethane (CBrCl3) has been determined by means of X-ray powder diffraction and thermal analysis techniques from 200 K to the liquid state. From the continuous series of mixed crystals, three relations of isomorphism have been evidenced: (i) between the stable low-temperature monoclinic (M) phases, (ii) between the stable orientationally disordered (OD) rhombohedral (R) phases, and (iii) between the stable OD face-centered cubic (FCC) of CBrCl3 and the metastable FCC phase of CCl4. The existence of such a metastable phase gives rise to the appearance, for a large range of composition, of metastable FCC mixed crystals that behave monotropically with respect to the R mixed crystals accordingly with the well-known monotropic character of the R to FCC phase transition of the CCl4. The isomorphism relationships i and ii enable us to validate the previously proposed structure of the monoclinic phase of CBrCl3 (C2/c, Z = 32) and to determine the lattice symmetry of the OD rhombohedral phase (relation ii) of CBrCl3, the symmetry of which is the same as that of the R phase Of CCl4, respectively. The thermodynamic assessment reproduces coherently the phase diagram (stable two-phase equilibria, M + R, R + L, and a part of the FCC + L) as well as the partially metastable [FCC + L] two-phase equilibrium and provides a coherent set of data for the thermodynamic properties of nonexperimentally available phase transitions of pure components.