Thermal reactions of Co/S$i_{0.76}$ G$e_{0.24}$ and Co/S$i_{0.54}$ G$e_{0.46}$ systems are studied. Gr segreation appeared at temperatures above 20$0^{circ}C$ even no cobalt silicides and/or cobalt germanosilicides were formed. At a temperature of 250-50$0^{circ}C$ Co(S$i_{1-y}$ G$e_y$) was formed, in which the Ge concentation was deficient and inhomogeneous. Ge played a crucial role in retarding the interfacial reactions between Co and S$i_{1-x}$G$e_x$ layer. At temperatures above 45$0^{circ}C$ the island structure was formed and the Ge concentration was enriched in the surface of the exposed S$i_{1-x}$ G$e_x$ layer. At temperatures above 53$0^{circ}C$ nearly Ge-free CoS$i_2$was formed. An interposing Si layer between Co and the S$i_{1-x}$G$e_x$ layer can alleviate the strain relaxation of the S$i_{1-x}$G$e_x$ layer during annealing.