The formation Of MoO3 and its spontaneous spread over an Au (111) surface have been studied by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Metallic Mo clusters grown by MO(CO)6 chemical vapor deposition (CVD) have a constant size independent of the Mo coverage. Molecular oxygen does not react with low coverage of Mo, probably due to the encapsulation of the Mo clusters by Au. At higher coverage, O-2 reacts with Mo, partially transforming the metallic Mo to Mo4+. NO2 can oxidize Mo efficiently to Mo6+ and Mo5+ species at all coverages investigated. XPS experiments show that the integrated intensity of the Mo 3d peaks increases by a factor of 2 upon the oxidation, suggesting the spread of the MoO3 over the surface. The STM study confirms this suggestion and provides the mechanistic details of the spreading. Mo oxide forms ramified two-dimensional islands covering a substantially larger fraction of the Au surface than the metallic Mo. We propose that the morphology change starts with the diffusion of oxide clusters (ramified-cluster-diffusion mechanism), followed by their breakdown to highly disordered two-dimensional islands of molecular MoO3.