Grain‐boundary and lattice diffusion of zinc vapor into copper at 550°C were measured with the electron‐probe microanalyzer and the results were used to calculate the diffusion coefficients. The lattice diffusion coefficientDLincreases from 5×10−13to 3×10−11over the concentration range from 2 to 27.5 atomic percent zinc. Over the same concentration range, the grain boundary diffusion coefficientDBchanges from 1×10−7to 7×10−6. The ratio of grain boundary diffiusion coefficient to lattice diffusionDB/DLremains at approximately 106.