The diffusivities of C and O in calcite were determined in a pure CO2 atmosphere at 100 MPa and temperatures ranging from 600 to 800 °C. The calcite crystals were preannealed and H2O was excluded from the system to determine the self-diffusion coefficients. The CO2 consisted of 99% 13C and 90% 18O. After heating for 7–147 d, diffusion profiles were measured with the use of secondary ion mass spectrometry. The results indicate that the diffusivity of C is DC = 7.77 × 10−9 exp (−166 ±16 kJ/mol/RT) cm2/s and of O is DO = 7.5 × 10−3 exp (−242 ± 39 kJ/mol/RT) cm2/s. In comparison with other determinations of diffusivities in calcite, diffusion of O under the experimental conditions is consistent with vacancy migration in the intrinsic region, and diffusion of C seems to occur by diffusion of carbonate anions. Increased pressure appears to reduce the activation energy and the value of D0, and the presence of H2O greatly increases the diffusivity of O without appreciably changing the activation energy. Closure temperatures calculated for isotopic exchange by diffusion predict that C isotope compositions of calcite are preserved during cooling in most geologic environments, but that O isotope compositions in H2O-rich environments are preserved only in rapidly cooling environments, such as contact metamorphic aureoles.