Hydrous ruthenium oxide powders annealed at different temperatures were studied by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction. The transition from the amorphous to the crystalline state was followed by recording the Ru XP-spectra. The peak shape of the O 1s XP-spectra provides information about composition and morphology. Comparison of the water amount obtained from deconvolution of the O 1s signal with the one calculated from gravimetric analysis demonstrates that the physically adsorbed water in the amorphous material evaporates under ultrahigh vacuum conditions. The observed decrease of chemically bound water with increasing annealing temperature is attributed to an increase of particle size.