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The reaction of H2O dissociation on Cu nanoparticles prepared by an atomic layer epitaxy (ALE) technique is discussed in this article. The activation energy of H2O dissociation, desorption energy of H2O, active sites for H2O adsorption, and structural changes of the Cu surface were studied using temperature-programmed desorption (TPD), temperature-programmed reduction (TPR), in situ IR spectroscopy, and X-ray absorption spectroscopy (XAS). The reduced Cu nanoparticles of the ALE-Cu/SiO2catalyst possess a slightly positive charge (Cuδ+) due to the effect of the nanosized particles. The very low activation energy of H2O dissociation (23 kJ/mol) and the exothermic dissociation heat were obtained from a series of H2-TPR experiments on Cu nanoparticles. It is suggested that the Cu nanoparticles might be partially oxidized to Cu2O, while some oxygen atoms are proposed to be located on the surface of the Cu particles over the course of water adsorption.