The diffusivity of C and O in calcite in mixed CO2-H2O fluid was determined over the range in xCO2 from 1.0 to about 0.2 at 700 °C, 100 MPa, with selected experiments conducted at pressures to 250 MPa and temperatures of 600 and 800 °C. The diffusivity of C, DC, varies little with xCO2, although there is some evidence for a slight increase in DC from ~5 × 10−18 to ~5 × 10−17 cm2/s with decreasing xCO2. Our data and those of others are consistent with a model for DC ∝ 1/fCO2. Despite the large uncertainty, we observed that the diffusivity of O, DO, increases from ~2 × 10−16 to ~5 × 10−14 cm2/s with xCO2 decreasing from 1.0 to 0. There is a good correlation at 700 °C between log DO and log fH2O regardless of the total pressure, matching the observations of previous workers. The data are consistent with a simple two-component model for the diffusion of O in calcite, one component for diffusion in the presence of CO2 and one in the presence of H2O: DO = DOCO2 + DOH2OaH2O. The activity of H2O is relative to the fugacity at 100 MPa, 700 °C. DOCO2 is 3.45 × 10−16, and DOH2O is 3.8 × 10−14 cm2/s. The data indicate that the rate of diffusion of C and O in calcite is controlled by reactions at the surface of calcite. Adsorption of H2O and the creation of vacancies at the surface account for the dependence of the diffusivity on the fugacity of the fluid components. There is little evidence that H itself diffuses into calcite. With this model and the values of DO in pure CO2 (Labotka et al. 2000) and in pure H2O (Farver 1994), the value of DO is predicted over the temperature range 600–800 °C and pH2O up to 300 MPa, the range of the data. Calculated closure temperatures for diffusive exchange of O between calcite and fluid are reduced by about 150 °C in the presence of an aqueous fluid.