The presence of large amounts of CO2 in geothermal fluids is shown to generally enhance convective instability. The buoyancy of water-CO2 solutions can be dominated by the large volume changes that occur when CO2 enters or leaves solution. The transfer of water between the liquid and vapor phases which accompanies the movement of CO2 into or out of solution also contributes to changes in the fluid volume and resultant buoyancy. The effective thermal expansivity &agr;eff in the CO2 rich fluid of the Broadlands geothermal region of New Zealand exceeds the coefficient of thermal expansion of ordinary water &agr;l for depths are large as a 2 km; &agr;eff is more than an order of magnitude larger than &agr;l at depths less than 500 m.