Interfacial reactions, phase equilibria and elecrrical properties of Co films on (001) oreinted GaAs substrate, in the temperature range 300-$700^{circ}C$ for 30min. have been investigated using x-ray diffraction and Augger electron spectropcopy. Cobalt started to react with GaAs at 38$0^{circ}C$ by formation of Co$_{2}$GaAs phase. At 42$0^{circ}C$, CoGa and CpAs nucleated at the Co and Co$_{2}$GaAs interface and grew with Co$_{2}$GaAs upto 46$0^{circ}C$. contacts produced in this annealing regime were rectifying and Schottky varrier heights increased from 0.688eV(as-deposite state) up to 0.72eV(42$0^{circ}C$). In the subsequent reation, the ternary phase started to decompose and lost stoichiometry at 50$0^{circ}C$. At higher temperature, Co$_{2}$GaAs disappered and CoGa/CoAs/GaAs layer structures were formed. Contacts produced at higher temperature regime(>50$0^{circ}C$) showed very low effective barriers. The results of interfacial reactions can be understood from the Co-Ga-As ternary phase diagram.