The structural, lattice dynamical and thermodynamic properties of copper halides CuCl and CuBr and their ternary alloys ¹CuCl_x-1Br_x have been studied using first principles calculations. The density functional perturbation theory (DFPT) and the virtual crystal approximation² (VCA) are employed. The variation of the structural parameters, the optical and acoustic phonon frequencies at the high symmetry points, the electronic and static dielectric constants, the Born effective charge are studied as a function of the concentration (x). Using the quasi harmonic approximation the thermodynamic functions : free energy, thermal expansion and specific heat are evaluated.