The thermal stability of gold nanoparticles (2.5-4 nm diameter) on fully stoichiometric TiO2(110) rutile substrates in an O-2 atmosphere and in air at up to ambient pressure, and for comparison under ultrahigh-vacuum (UHV) conditions, was investigated by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Substrate material and annealing parameters were chosen similar to those used for the pretreatment of realistic disperse Au/TiO2 catalysts. Under UHV conditions, the particles are rather stable even after annealing at 400 degreesC, and only minor changes in the particle size are detected after annealing at 500 degreesC. In 50 mbar O-2, the particles are relatively stable at room temperature but grow significantly after annealing to 400 degreesC. In air, particle growth starts already at room temperature, accompanied by the formation of Au-OH species. The relevance of these results for the understanding of the pretreatment of disperse Au/TiO2 catalysts is discussed.