It is shown that the difference in the free energy of activation for atoms diffusing in the lattice and in the grain-boundary is precisely the absolute grain-boundary energy. The self-diffusion data in Au in the lattice and along the grain-boundaries yielded excellent values of grain-boundary energies as a function of temperature. An addition of only 1·2 at. % Ta to Au was found to reduce the grain-boundary energies rather drastically. From the lowering of the grain-boundary energy, the solute excess at the grain-boundary was determined as a function of temperature which varied by over an order of magnitude in the 204°394°C range. An interaction energy of 5·6(±0·5) kcal/mole and entropy of 3·0 cal/mole K are determined for the Ta segregation on to the arbitrary high-angle grain-boundaries in Au. The grain-boundary energies and their solute interaction are discussed in the light of the available atomic models. © 1976 Taylor & Francis Group, LLC.