The effect of the thickness of the SiGe layers in the Co-SiGe/Si reaction is investigated. Formation of the disilicide phase is retarded when compared to the reaction of Co with pure Si substrates. This delay is dependent on the thickness of the SiGe layer, which determines the effective Ge concentration at the reaction front, changing the energetics of the reaction. Besides, Ge expelled from the Co(Si1-yGey) compound blocks the Co diffusion paths, slowing down the reaction. The thermal stability of the silicide layer is also affected by the Ge segregation, with the film agglomerating at lower temperatures when formed on thicker SiGe layers. The slow supply of Co atoms to the reaction front, due to the blocking of Co diffusion paths by the segregated Ge, is believed to be the reason for the presence of the preferential (h00) orientation of the disilicide phase observed in our samples.