To contribute to a better understanding of point defects and defect-related transport properties in cobalt orthosilicate, Co2SiO4, the orientation-dependent diffusion of cobalt in high purity, synthetic single crystals of Co2SiO4 being in thermodynamic equilibrium with silica was experimentally studied as a function of crystallographic orientation, oxygen activity and temperature. The oxygen activity dependence of the diffusion of cobalt in Co2SiO4 along the three principle orientations at 1300 degrees C is at high oxygen activities compatible with cobalt vacancies and holes as majority defects. At lower oxygen activities the oxygen activity dependence of the cobalt tracer diffusion coefficients becomes smaller than at high oxygen activities. This is most likely related to an increase of the concentrations of cobalt interstitials at lower oxygen activities and at such oxygen activities also to an increased influence of the impurity ions being present in the samples used. The latter is due to smaller overall point defect concentrations at lower oxygen activities compared to those found at higher oxygen activities. The temperature dependence of the cobalt tracer diffusion along the orientations denoted above was investigated in the range between 1200 and 1300 degrees C at a(O2) = 1 (a(O2),= P-O2/P degrees(O2) and P degrees(O2) = 1 atm). The results obtained suggest that the anisotropy of the diffusion of Co in Co2SiO4 does not very significantly vary with the temperature. When using the space group Pbnm when assigning crystal orientations the ratio found for the cobalt tracer diffusion coefficients at a(O2) = 1 is approximately D*(Co(001)): D*(Co(010)):D*(Co(100)) = 30:3:1. (C) 2012 Elsevier B.V. All rights reserved.