The non-equilibrium grain boundary co-segregation of boron and magnesium in Ni3Al-B-Mg alloys was determined by Auger electron spectroscopy (AES) in conjunction with ion sputtering in a cooling rate range of 0.05-269 K/s from temperatures of 1023, 1223 and 1373 K. The analytical expressions of diffusion rate equations describing non-equilibrium segregation process based on the concept that mobile solute-vacancy complexes migrating to grain boundaries is responsible for non-equilibrium grain boundary segregation of solute atoms in an alloy was used to simulate well the experimental results. The diffusion coefficients for boron atoms, boron-vacancy complexes, magnesium atoms and magnesium-vacancy complexes were determined, and the binding energy of boron-vacancy and magnesium-vacancy complexes was estimated. (C) 2003 Kluwer Academic Publishers.