The effect of pressure on silicon diffusion in San Carlos olivine has been determined using a uniaxial split-sphere apparatus (USSA-2000) and the nuclear reaction analysis technique (NRA) on the 30Si isotope. Experiments were performed at high temperature, T=1763 K, and pressures between 4 and 9 GPa. The specimens were inserted into a pure Fe capsule, which is very effective in maintaining the oxygen fugacity within the stability field of olivine, as well as providing a soft medium to mechanically protect the crystals. Diffusion profiles along the b&vec; crystallographic axis and of characteristic length of the order of 50 nm were obtained after annealing the olivine samples between 1 and 4 hours. We find the activation volume for silicon diffusion in San Carlos olivine to be VSi=+(0.7¿2.3)¿10-6 m3/mol after a correction for oxygen fugacity which is pressure-dependent. This result demonstrates that pressure has practically no effect upon silicon diffusion under our temperature and pressure conditions. Extrapolation of our high-pressure measurements to 1 atm gives a Si diffusion coefficient, log(DSi)=-18.9¿1.0 (with DSi in m2/s). Finally, our experiments show that according to the point-defect model of Jaoul <1990>, the activation volume for creep of olivine at high temperature is dominated by the effect of pressure on the Mg defect concentration and has an activation volume close to 6¿10-6 m3/mol. ¿ 1999 American Geophysical Union