Because of the relatively large surface energy of liquid basalt, the nucleation of CO2 bubbles in mid-ocean ridge basalts (MORB) requires degrees of supersaturation, which vary from 1.5 to 7, depending on the pressure. This pressure sensitivity is a direct consequence of the increase in the molar volume of gaseous CO2 when the pressure decreases. This causes the surface area of stable nuclei with a constant number of carbon dioxide molecules to become larger with decreasing pressure. Of the major igneous gases, CO2 is the most abundant in uncontaminated MORB, but has the smallest solubility. Therefore bubbles in MORB at P>100 bar are CO2 dominated. The recently discovered CO2 supersaturation in MORB et al., 1988; Fine and Stolper, 1986> can be explained as being due to the difficulty of bubble nucleation at low pressure. ¿ American Geophyscial Union 1990