The effects of interdiffusion and microstructure on the hardness and resistivity of Cr/Cu/Co/Au thin films were studied at temperatures ranging from 25 to 600 °C. Cobalt was found to be a very effective diffusion barrier between Cu and Au at temperatures of ≤400 °C for annealing times greater than 2 h. As a result, little or no change in hardness or resistivity of the Cr/Cu/Co/Au structure occurs. Above 400 °C, the Co layer breaks down, and there is massive interdiffusion of Cu and Au, which is associated with grain growth, grain boundary precipitation and surface roughening of the films. The observed changes in the hardness and resistivity correlate with the degree of interdiffusion of Cu and Au for temperatures ≤500 °C. The activation energy for interdiffusion is 1.65 eV and that for Au diffusion into Cu via the Co layer is 1.91 eV. These values are consistent with those reported for grain boundary diffusion in thin films.