It is probable that elemental carbon exists in the source region of a basaltic magma and is suspended in the magma during ascent. As carbon has a large redox capacity, it is probably in control of the magmatic fO2 from the source region to the deep crustal environment. Isothermally carbon becomes more reducing with decreasing pressure, and thus reduces the host magma upon ascent. If a magma is anhydrous (e.g., lunar basalts), the reduction by carbon continues through the extrusive phase and the relative fO2 decreases rapidly until buffered by the precipitation of a metallic phase. If a magma is hydrous (e.g., terrestrial basalts), reduction by carbon is eventually superceded by oxidation due to the loss of H2, which is generated by the reaction of C H2O and also by the thermal dissociation of H2O, and ferric iron is produced. Cumulus crystallization of ferrous silicates also contributes to the oxidation of magma. The relative fO2 of a hydrous magma initially decreases as the magma ascends from the source region, and then increases until magnetite crystallization curbs the rising trend of the relative fO2.