Abstract Reactions at the interface between Ta and GaAs have been studied at 850–950 °C with thick film Ta/GaAs diffusion couples in the region of unlimited material supply. The development of a reaction layer is observed, the layer being composed of phases that can be identified in the ternary TaGa-As phase diagram. The growth kinetics of this layer are parabolic with different reaction mechanisms at 850 and 900 °C. The different reactions can be understood from the experimentally determined ternary phase diagram and from the thermodynamic driving forces for the various competing phases in the reaction layer. Data on reactions in thin film Ta/GaAs contacts can be interpreted on the same basis. The thermodynamic stability of intermetallic phases in this system for use as contact materials on GaAs is also discussed, and TaAs is identified as the most appropriate phase.