The Gibbs Ensemble Monte Carlo (GEMC) simulation method has been used for vapor-liquid equilibrium calculations of the binary systems R22-R142b and R22-R152a, which are considered to be promising mixture refrigerants for replacing R12. All the molecules have been treated in terms of an effective Lennard-Jones potential, with temperature dependent parameters regressed by fitting vapor pressures and saturated liquid densities at various temperatures for pure substances of interest. Good agreement between experimental and calculated data from the volume-translation Peng-Robinson equation of state and our simulated results, including the compositions, molar volumes for both the vapor and liquid phases, and heats of vaporization, indicates that the GEMC method can be applied to the description of phase behavior for these CFC alternative binary systems.