Specific contact resistance of gold metallization on In1-xGaxAsyP1-y has been measured as a function of composition from InP to In0.53Ga0.47As. The alloy formation of the contacts was investigated in the temperature range 130°-550°C, using temperature dependent in situ x-ray diffraction. The thermodynamics of the solid state reactions are discussed. The electrical contact behavior can be explained in terms of the alloys which form at various temperatures. In the ternary Au/InGaAs/InP contact polycrystalline GaAs and a thin layer of Au3In2 form at lower temperatures (150°-250 °C) and remain stable up to 550 °C. This is consistent with the measured low contact resistance which is comparable to that of the Au/GaAs contact.