The solution of H2O, CO2, and H2O–CO2 mixtures in silicate magmas has many effects. Fusion temperatures are lowered, and the fusion and crystallization sequences may be modified. Hydrous minerals and carbonates may become stable within the fusion interval. Liquid compositions are changed as a function of dissolved volatile content, and as a function of H2O/CO2. The physical properties of magmas may be influenced markedly by dissolved volatile components. The presence of vapour bubbles arising from exsolution of volatile components during uprise or crystallization of magma may be responsible for physical and geochemical effects. These effects are outlined by examination of the phase relationships in several synthetic systems and rock–H2O–CO2 systems. The results demonstrate that H2O is more influential than CO2 in magma generation at crustal pressures. The presence of CO2, however, may influence the evolutionary history of a crustal magma body. In the mantle at depths of 75–80 km, the influence of CO2 becomes as significant as that of H2O because CO2 reacts with peridotite stabilizing calcic dolomite, and the solubility of CO2 increases dramatically. The compositions of near-solidus mantle magmas are strongly influenced by H2O/CO2 and by the distribution of carbonate, amphibole and phlogopite in the peridotite. The normal product of crustal anatexis is H2O-under-saturated granitic liquid. Tonalite liquids can-not be produced by crustal anatexis during normal regional metamorphism.