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Using vibrational spectroscopy and SIMS, we determined the solubility and speciation of C–O–H–N dissolved volatiles in mafic glasses quenched from high pressure under reduced conditions, with f O 2 from −3.65 to +1.46 relative to the iron–wustite buffer (IW). Experiments were performed on martian and terrestrial basalts at 1.2 GPa and 1400 °C in graphite containers with variable availability of H 2 O, and in the presence of FePt alloys or Fe–C liquids. The dominant C–O–H–N species varies systematically with f O 2 and H 2 O content: the carbonate ion prevails above IW + 1, but for dry conditions between IW−2 and IW + 1, C O species are most important. Below IW, reduced NH-bearing species are present. At the most reducing and hydrous (∼0.5 wt% H 2 O) conditions, small amounts of CH 4 are present. Concentrations of C diminish as conditions become more reduced, amounting to 10 s to ∼100 ppm in the interval ∼IW−2 to IW + 1 where C O species dominate, and as little as 1–3 ppm at more reduced conditions. Concentrations of non-carbonate carbon, dominated by C O species, correlate with CO fugacities along a trend implying that the species stoichiometry has just one C O group and suggesting that carbonyl complexes (transition metals with multiple carbon monoxide ligands) are not important species under these conditions. C partition coefficients between Fe–C liquid and silicate melt increase with decreasing f O 2 , becoming as great as 10 4 for the most reducing conditions investigated. The low solubility of C in silicate liquids under reducing conditions means that most C during the magma ocean stage of planetary differentiation is either segregated to the core or in the overlying atmosphere. Precipitation of C-rich phases in a carbon-saturated magma ocean is also possible, and is one mechanism by which some C can be retained in the mantle of a planet. The predominant magmatic carbonaceous species for both martian and lunar volcanism is likely C O.