The lattice diffusion coefficient of copper in high‐purity nickel single crystals is measured between 775° and 1050°C. The diffusion coefficient satisfies the equationD=0.27 exp(−61.02/RT) cm2sec−1, with an error inD0of ±22% and inQof ±1%. A comparison with the results of other authors is given, showing that the dc measured is considerably lower than in previous investigations. This effect is referred to grain‐boundary diffusion and chemical impurities in the specimens used earlier.