Interfacial reactions of cobalt with differently doped Si0.8Ge0.2 layers epitaxially grown on silicon, during furnace annealing up to 600 °C, were compared. Undoped and highly boron- or antimony-doped Si0.8Ge0.2 strained layers were used for this study. The analytical tools employed were Auger electron spectroscopy, secondary ion mass spectroscopy, x-ray diffraction, and transmission electron microscopy. Both dopants accumulate at the surface as a result of the interfacial reaction with Co but boron also remains trapped between the reacted Co monosilicide region and a thin Co-rich near-surface layer. In addition, the boron-doped sample exhibits a large accumulation of Ge at the interface between the unreacted SiGe and the reacted monosilicide. The latter effect is related to the loss of Ge from the unreacted SiGe layers observed after high-temperature annealing, the B-doped SiGe epilayer undergoing the largest loss at 600 °C, while the Sb-doped epilayer was only slightly affected.