The phase diagram of the two-component system 2-methyl-2-chloropropane ((CH3)(3)CCl) and 1,1,1-trichloroethane ((CH3)CCl3) has been determined by means of X-ray powder diffraction and thermal analysis techniques from 180 K to the liquid state. The isomorphism relationship between the stable orientationally disordered (OD) face-centered cubic (FCC) phase of (CH3)(3)CCl and the metastable OD FCC phase (monotropic behavior with respect to the OD rhombohedral stable phase) of (CH3)CCl3 has been put into evidence throughout the continuous evolution of the lattice parameters and the existence of the two-phase equilibrium [FCC + L] (L = liquid) for the whole range of composition. This equilibrium interferes with a rhombohedral (R) plus liquid ([R + L]) equilibrium, giving rise to a peritectic invariant. The validity of the well-known crossed isodimorphism formalism has been experimentally demonstrated, and a subsequent thermodynamic assessment has been performed to obtain the excess properties of the OD FCC and R phases. In addition excess properties of the R and FCC phases are compared, and the existence of short-range orientational order for the R phase is inferred.