Point defects and defect-related transport properties of transition metalcontaining orthosilicates with the olivine structure are interesting topics but are not yet well understood. At high temperatures, the transport properties of sufficiently pure olivines are governed by point defects. To improve the currently limited understanding of the defect structure and defect-related transport properties of olivine group compounds, the transport of matter in orthosilicates of the type Me2SiO4, with Me = Co and Mn, was experimentally investigated. The cation tracer diffusion of cobalt and manganese in cobalt and manganese orthosilicates, respectively, was studied as a function of crystal orientation, oxygen activity and temperature using high purity, synthetic cobalt and manganese orthosilicate single crystals grown by the floating zone method. Modeling of the observed oxygen activity dependancies of the cation tracer diffusion coefficients and of point defect concentrations was performed based on data obtained from this study in conjunction with other defect-related data reported in literature. The oxygen activity dependence of the diffusion of cobalt in Co2SiO4 along the three principle orientations at 1300 ° at high oxygen activities is compatible with C 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 higher oxygen activities, which is most likely related to an increase in concentrations of cobalt interstitials. When using the space group Pbnm for assigning crystal orientations, the ratio found for the cobalt tracer diffusion * * * coefficients at aO2 = 1 is approximately DCo[001]:DCo[010]:DCo[100] = 30:3:1. The oxygen activity dependence of the diffusion of manganese in Mn2SiO4 along the three principle orientations at 1200 ° is, at high oxygen activities, C compatible with manganese vacancies and holes as majority defects. The observed oxygen activity dependence of the diffusion of manganese in Mn2SiO4 also suggests that at high oxygen activities, high mobility minority defects, which are most likely associates formed between holes and manganese vacancies, also contribute very significantly to the manganese tracer diffusion. At lower oxygen activities, the observed weaker oxygen activity dependence of the Mn tracer diffusion coefficient compared to that at higher oxygen activities can be attributed to an increased concentration of manganese interstitials and their increased contribution to the transport of Mn in Mn2SiO4. A similar observation was found for the diffusion of Co in Co2SiO4 at lower oxygen activities. The ratio found for the manganese tracer diffusion coefficients at log aO2 = &2.7 is approximately * * * DMn[001]:DMn[010]:DMn[100] = 5.3:2.2:1.