We combined scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) to study the molecular and electronic structure of submonolayer tungsten oxide supported on anatase TiO₂(101), −(001), and rutile TiO₂(110) surfaces. We found that monomeric tungsten oxide species form on all three TiO₂ surfaces upon mild annealing at 400 K, with a geometry depending on the supporting facet. At ∼600 K, surface diffusion of the monomers sets in, but the monomers remain on the surface without diffusing into the bulk even at higher annealing temperatures. As-deposited tungsten oxide at monolayer coverage is stronger oxidized than thick layers. At elevated temperatures (400-900 K), significant reduction is observed, strongly dependent on the TiO₂ facet employed and bulk defects within the substrate. Among the TiO₂ surfaces studied, the weakest reduction by vacuum annealing was found for tungsten oxide supported on anatase TiO₂(001).