Inelastic neutron scattering (INS) has been used to study the adsorption of water, at different concentrations, in H-ZSM-5. INS is the only vibrational technique where the intensities can be calculated with reasonable accuracy from atomic displacements. This feature is used here to simulate the INS spectra of the two possible structures resulting from water interaction with the Bronsted acid sites of the zeolite : hydrogen-bonded water or hydroxonium ion. The atomic displacements for the two structures are derived from recent ab initio MP2 calculations (Krossner, M.; Sauer, J. J. Phys. Chem. 1996, 100, 6199-6211). The comparison between experimental and calculated INS spectra confirms that the first water molecule is attached to the acid site via two hydrogen bonds, in agreement with the conclusion made by Krossner and Sauer. Hydroxonium ions are not found in H-ZSM-5; however, this protonated species might be present in zeolites with a different structure.