The vapor pressures of magnesium over a series of two-phase alloys of Mg with Ce, Nd, Gd, Dy, Er, and Lu have been measured by the Knudsen effusion technique in the temperature range 650 to 930 K. These vapor pressure measurements were combined with data concerning the terminal solubility of magnesium in the rare earths and the vapor pressure of pure magnesium to evaluate the standard free energies of formation of the twenty-four intermediate phases in these six binary systems. Trends in the data correlate with the number of unpaired 4f electrons of the lanthanon component, and the inference is that the 4f electrons make a contribution to the bonding interactions. There is also evident a shift in the pattern of the free energies of formation of the phases with the shift occurring as the atomic number of the lanthanon component is increased. This implies modification of the bonding interactions by some additional factor which might be either a size factor or a crystal-environmental dependence of the strength of the 4f bonding contributions. Experiments are proposed to distinguish between the possibilities.