Phase transitions that involve solid‐state reactions between cobalt and thin films of germanium have been investigated. Germanides are formed by reacting Co (300 A˚ thick) with thin layers of Ge (∼2000 A˚ thick) deposited on silicon substrates. Germanium was deposited on Si by rapid thermal chemical‐vapor deposition and cobalt was deposited onto Ge by evaporation. The Co/Ge/Si stacked structure samples were then rapid thermally annealed at atmospheric pressure in an inert ambient consisting of Ar. Using x‐ray‐diffraction spectroscopy, Co5Ge7and CoGe2are identified as the phases which form at 300 and 425 °C respectively. The sheet resistance was found to be a strong function of the annealing temperature and a minimum resistivity of approximately 35 &mgr;&OHgr; cm is obtained after annealing at 425 °C. The minimum resistivity material corresponds to the CoGe2phase with an orthorhombic crystal structure. Above 600 °C, the resistivity increases due to an instability of the solid‐phase reaction between Co and thin Ge layers deposited on Si. This instability is attributed to rapid Co diffusion at the temperatures which are required to form CoGe2along with structural defects in the Ge layer.