In order to test the feasibility of hypotheses of vapor transport as explanations of ore deposition, an attempt is made to calculate the composition of a vapor phase in equilibrium with a cooling intrusive at 600 degrees C. The proportions of the major constituents are estimated from Shepherd's analyses of gases obtained by heating igneous rocks, from Rubey's calculations of the total amount of volatile material given off by the earth, and from requirements of thermodynamic equilibrium with the minerals of intrusive contacts. Concentrations of volatile metals and metallic compounds in the vapor are calculated from thermodynamic data, assuming that the vapor is saturated with each metal and that equilibrium is maintained with the common metallic minerals found in contact-metamorphic zones and high-temperature vein and replacement deposits. For most metals the chlorides are the most abundant compounds in the vapor, but volatilities of the chlorides differ from simple vapor pressures because of equilibrium requirements.Neither sulfides, oxides nor native metals would have sufficient concentrations in the vapor to play a significant role in ore deposition. The lack of correlation between melting points of metals, their vapor pressures, and their maximum possible amounts in the vapor is proof that metallic melting points cannot be correlated with "mobilization" of the metals.Many common metals are present in magmatic vapor at 600 degrees in sufficient quantity to account for the formation of ore deposits. Differences in calculated volatilities provide a good explanation for the restriction of mercury and antimony to low-temperature deposits, and for the appearance of lead later than zinc in zonal and paragenetic sequences. Nevertheless, simple vapor transport cannot be a complete explanation for the origin of ore deposits. Outstanding difficulties are the very low volatilities of copper, silver, and gold, and the fact that manganese has a lower volatility than lead.