Interfacial reactions of Pd and Pd/Si films on [001]-oriented GaAs substrates have been studied by X-ray photoelectron spectroscopy. In the as-deposited Pd/GaAs system, Pd interacts with Ga and the As is isolated. A Pd-Ga-As compound is observed to form under the Pd-Ga and As layers. Annealing the Pd/GaAs system at 450 degrees C leads to the formation of islands composed of Pd-Ga and Pd-As, amongst which a Pd-Ga-As compound forms. In the Pd/Si/GaAs system, Pd diffuses in to the GaAs substrate, the Pd-Ga and Pd-Ga-As compounds are formed. In the 450 degrees C-annealed Pd/Si/GaAs system, little interfacial reaction at the GaAs substrate occurs. Possible interfacial reactions in the Pd/GaAs and Pd/Si/GaAs systems, including the previously investigated Pt/GaAs, Pt/Si/GaAs, Ni/GaAs and Ni/Si/GaAs systems are considered by calculation of the change in enthalpy of the reactions. The thermodynamic analyses agree with the experimental results.