The solid state reaction between copper and silicon has been studied in the temperature range 350 to 650°C. The rate-limiting step is the diffusion of copper through the main product Cu3Si. When pure copper is used, a bulk diffusion mechanism is operative above 470°C, and the activation energy is 175 kJ/mol. Below 470°C incubation times occur. The reaction proceeds by grain-boundary diffusion, with an activation energy of 110 kJ/mol. When phosphorus-doped copper is used, bulk diffusion of copper through Cu3Si occurs above 530°C. Below this temperature grain-boundary diffusion occurs with an activation energy of 92 kJ/mol. No incubation times were observed. The segregation of phosphorus at the reaction interface removes the reaction barrier. Cu15Si4 and Cu5Si seem to be absent from the diffusion couples because of kinetic factors.