We have determined the self diffusion of C and O in calcite over the temperature range of 600-800 °C at pressures ranging from 0.1 to 200 MPa. Single crystals of calcite were heated in cold-seal hydrothermal vessels for periods ranging from 7 to 147 d. The crystals were encapsulated in platinum with ¹³C¹⁸O₂. After heating, we measured the profiles of ¹³C/(¹³C + ¹²C) and ¹⁸O/(¹⁸O + ¹⁶O) by sputtering the sample in the SIMS ion microprobe. In most samples, diffusion profiles were limited to within 1 µm of the surface. The diffusivity of O at 800 °C changes little over the investigated pressure range and has a value of ~10^-14 cm²/s. At pressures greater than ~25 MPa and at 800 °C, the diffusivity of C decreases slightly with increasing pressure, decreasing from 3 x 10^-16 cm²/s at 25 MPa to 6 x 10^-17 cm²/s at 200 MPa. The value of [ (log D_C)/ p]_{800 °C} is -0.004 log (cm²/s)/MPa. At low pressure, the diffusivity of C at 800 °C is substantially greater than at high pressure. At 0.1 MPa and 800 °C, the diffusivity of C is comparable to that of O. The measured C diffusion profiles at low pressure, though, are complex in detail, as seen in the figure. Carbon seems to undergo a change in mechanism of diffusion at a pressure corresponding to near surface environments.