CO2 is commonly produced during steamflooding. In the higher temperature steamfloods the amount of CO2 produced is such that the only viable source is carbonate minerals. It is shown by reference to CO2 concentrations in natural geothermal systems that in most common rocks and sediments, CO2 will be naturally generated under hydrothermal (~300°C) conditions by the dissolution of carbonate minerals. The solubility of CO2 is a strong positive function of temperature and, at 300°C, is more than sufficient to account for high CO2 concentrations such as those encountered at Chevron's Buena Vista Hills (B.V. Hills) steamflood pilot test. A linear kinetic model Is developed to describe chemical equilibration and the generation of CO2 in the hot parts of the reservoir. The model also describes the precipitation of CO2 in the cooler parts of the rock formation where the fluid moves ahead of the thermal front. Changes in Carbon isotope concentration are included in the model. The small difference between the 13C in the produced gas and the source carbonates at B.V. Hills suggests little CO2 is precipitated as carbonate within that reservoir – a prediction that can be tested by post-steamflood coring.