Interfacial reactions of Co and Si/Co films on GaAs have been investigated using X-ray diffraction(XRD), Auger electron spectroscopy(AES), cross-sectional transmission electron microscopy(XTEM), microdiffraction, and energy dispersive spectroscopy(EDS). Cobalt starts to react with GaAs at 380°C by formation of ternary phase, most probably Co2GaAs. At 420°C, CoGa nucleates at the Co and Co2GaAs interface and grows with Co2GaAs. At higher temperature, Co2GaAs disappears and CoGa/GaAs layer structures are formed. Annealing of the layer-deposited Si/Co films at 380°C results in the formation of Co2GaAs. At 420°C, the entire layer of Co is consumed, while Co2Si transforms to CoSi, and binary phases, CoGa and CoAs, are formed on top of the ternary phase beneath the CoSi layer. In the subsequent reaction, CoSi grows at the expense of the decompositions of CoGa and CoAs at 460°C. At 600°C, ternary phase is decomposed, and CoSi forms interface with GaAs. This finding can be understood from the calculated Si-Co-Ga-As quaternary phase diagram.