Au nanoparticles and Au films for growth of nanowires on the GaAs111B surface have been studied by a combination of experimental and theoretical techniques. If Au is present in either form, annealing to temperatures relevant for nanowire growth results in the formation of a thin Au wetting layer. Based on density functional theory calculations and experimental data, a structural model is proposed with an Au atom on every third threefold hollow hcp site of the Ga lattice. The authors observe that the stability of Au nanoparticles is governed by the presence of the wetting layer and outdiffusion of Au from the nanoparticles.