We report in this Letter the measurement of unusually well-defined vibrational modes of water. Calcium chloride forms complexes made of different amounts of water, namely 1/3, 2 or 4 water per CaCl2, molecule. Based on the crystallographic structure, we performed normal mode calculations from first principles, and simulated the neutron scattering spectra. A very good agreement between calculation and experiment, without any parameter refinement, confirms and completes the intuitive assignment of vibrational excitations of water. A closer took at the calculation enables us to investigate the anharmonicity of the system and the dispersion of the excitations along the hydrogen bonds. (c) 2005 Elsevier B.V. All rights reserved.