The thermal stability of Co silicide on single crystal and polycrystalline Si has been investigated. Co films were evaporated onto (100) Si and undoped polycrystalline Si and annealed in vacuum. Resulting silicide films were examined using Rutherford backscattering spectroscopy, scanning electron microscopy, electron—induced x—ray spectroscopy, and sheet resistivity measurements. We find that CoSi2 on single crystal (100) Si remains stable through 1000ºC. In contact with undoped polycrystalline Si, intermixing begins at temperatures as low as 650ºC for 30min annealing. The Co silicide and Si layers are intermixed after 750ºC 30min annealing, giving islands of Si surrounded by silicide material, with both components extending from the surface down to the underlying oxide layer. The behavior of CoSi2 contrasts with results reported for TiSi2 which agglomerates on single crystal Si around 900ºC but is stable on polycrystalline silicon as high as 800ºC. Resistivity measurements show that the Co silicide remained interconnected despite massive incursion by Si into the silicide layer.