Iron- and C-self diffusion have been measured in Fe3C composition liquids at 8 and 15 GPa. Diffusivities fall within the range of values for molten metals and scale inversely to the atomic radius of each species. This supports models such as the Stokes-Einstein relation and the free volume model which relate transport properties to the atomic radius. Along the melting curve, diffusivity is predicted to be constant and we tentatively predict outer core diffusivities at the inner core boundary of 5 ¿ 10-9 m2 s-1 and 7 ¿ 10-9 m2 s-1 for iron and carbon, respectively. This would correspond to a viscosity of around 15 mPas.