The solubility and diffusivity of C in olivine at 0.1, 2.0, and 3.0 GPa have been constrained by annealing single crystals of San Carlos olivine in 14C-labelled CO2, CO2-H2O, and graphite at 1200¿--1450¿C. In nearly all annealed specimens, carbonaceous material was observed at the diffusion interface that consisted of either (1) graphite used as a C source, (2) elemental C (probably graphite) formed by reduction of the C-O-H fluid phase present, or (3) carbon-rich surface films interpreted as silicate dissolved in the fluid phase at high pressure, quenched onto the crystal surface. The significant range of 14C-beta particles in olivine (and photographic emulsion) coupled with the presence of carbonaceous material at the diffusion interface gives rise to apparent concentration profiles. After removal of the carbonaceous material we retained no unambiguous evidence for C diffusion into olivine. The sensitivity and spatial resolution of the beta track method constrain either the solubility to be less than 30 ppmw (parts per million by weight) C or the diffusivity to be less than 10-12 cm2/s. Observations and geochemical data on natural rocks bearing on the nature and distribution of C in the Earth's upper mantle are discussed in light of these constraints. ¿ American Geophysical Union 1988