The solid state reaction between copper and germanium has been studied in diffusion couples in the range 400 to 575°C. The main product is coarse-grained Cu3Ge, but Cu5Ge and the (Cu, Ge) solid solution are also formed, albeit in thin layers. The reaction rate is determined by bulk diffusion of copper through the germanide layer, with an activation energy of 104 kJ/mol. Above 570°C a sudden acceleration occurs. At 500°C the interdiffusion coefficient in the solid solution is 6 × 10*(-12) cm2/s, D(Cu5Ge) = 4 × 108*(-11) cm2/s and 0D in Cu3Ge is raised from 1 × 10*(-10)cm2/s at 22 atom-% Ge to 1×10*(-8) cm2/s at 25 atom-% Ge. Contrary to the copper-silicon reaction, no reaction barrier has been observed, and so the presence of phosphorus in copper does not influence the reaction between copper and germanium.