The diffusion of boron, phosphorus, and arsenic in layers of CoSi2has been investigated. In order to limit effects due to interfaces and grain boundaries, the layers used were extremely thick, — 900 nm. The dopants were introduced via implantation to a depth of — 70 nm. Values of the activation energy for the lattice diffusion of both boron and phosphorus were obtained. In contrast to these two elements arsenic was found to be almost totally immobile. Some implications of these results for practical applications are briefly discussed. © 1988, American Vacuum Society. All rights reserved.