This paper presents evidence that the barrier heights of ruthenium Schottky contacts formed on n- and p-type GaAs are determined by the pinning of the Fermi level by interfacial antisite defects. The surface Fermi level of as-deposited Schottky contacts is believed to be pinned 0.5 eV above the valence band maximum, in agreement with the 0.89±0.06 eV barrier height measured for n-GaAs and the 0.50±0.03 eV barrier height measured for p-GaAs. Annealing of the ruthenium Schottky contacts decreased the barrier height for n-GaAs to 0.75±0.02 eV and increased the barrier height to 0.72±0.06 eV for p-GaAs. These changes in barrier height are attributed to a shift in the Fermi-level pinning position towards the conduction band minimum, which is ascribed to an interface reaction that increases the As_Ga:Ga_As ratio. Hydrogenation of the ruthenium Schottky contacts resulted in the neutralization of interface defects, thereby producing barrier heights that are instead determined by the ruthenium work function.