The adsorption of H2O on Ru(001) at temperatures of 95 K and 165 K has been investigated using electron energy loss spectroscopy, low-energy electron diffraction, and ultraviolet photoelectron spectroscopy. The vibrational data, together with the structural information available, supports a model in which aggregates of water molecules form via population of a first (chemisorbed) layer and subsequent hydrogen bonded layers. At the higher temperature of adsorption, 165 K, major differences in the vibrational spectra are apparent. Possible reasons for these differences are discussed.