The solubility of CO-CO 2 vapors in equilibrium with graphite has been measured in several silicate melts at pressures to 30 kbar and temperatures to 1700°C. The vapors contain 30–40% CO, by thermodynamic calculation. The CO-CO 2 vapors are slightly more soluble than CO 2 alone in melts investigated at near-liquidus temperatures. The temperature dependence of the solubility of CO is apparently negative, in contrast to the positive temperature dependence of CO 2. As a result, the solubility of CO-CO 2 at superliquidus temperatures is less than that of CO 2. Infrared spectra of quenched glasses containing CO and CO 2 essentially show only a CO 3 2− absorption band. The depressions of the liquidi of two compositions by CO-CO 2 vapor were found to be slightly greater than the depressions by CO 2 vapor. These phase relations imply that melting points are depressed more by CO than by CO 2 and that CO/(CO + CO 2) is somewhat greater in the liquid than in the vapor. The temperatures and pressures at which the liquidus phase of a peridotitic composition changes from olivine to orthopyroxene have been determined. These determinations suggest that melts containing CO and CO 2 are nearly as polymerized as melts containing only CO 2. These melts are significantly more polymerized than volatile-absent melts. Melting relations of peridotite in the presence of CO-CO 2 will be similar, accordingly, to melting in the presence of CO 2.