A mono-energetic positron beam is used to probe the Au/GaAs(110) interface by observing the Doppler broadening of the annihilation radiation as a function of beam energy. The observed data are fitted well by a three-layer model in which the intermediate layer absorbs positrons. The annihilation radiation from this region indicates that it possesses open volume defects. This intermediate layer is attributed to both the Au-Ga alloyed structure (of approximately 100 AA width) that forms close to the expected interface position as a result of atomic intermixing and an extended adjacent defected area in the An overlayer resulting from Ga and As outmigration. The nature of the defects in these regions is discussed. While evidence is found for Au induced dissociation of the GaAs lattice, the present data are not sufficiently sensitive to give any definite conclusion regarding the presence or absence of vacancy type defects in the near-interface region of the substrate.