Spectroscopic techniques in controlled atmosphere, such as solid-state H-1, H-2 and (31)p magic angle spinning nuclear magnetic resonance (MAS-NMR) and inelastic neutron scattering (INS) spectroscopies, have been used to investigate the effect of dehydration on structural modifications and acidic properties of solid 12-tungstophosphoric acid H3PW12O40 and its cesium salt Cs1.9H1.1PW12O40 Thermogravimetric analysis and XRD experiments gave complementary informations about proton/water contents and structure of the samples. H-1, H-2, and P-31 MAS-NMR spectra were recorded as a function of the degree of dehydration/rehydration and allowed one to characterize the protonic species present in the samples, such as OH groups and protonated clusters H+(H2O)(n). INS spectra, recorded at 4 K on samples dehydrated at 473 K, suggested the presence of hydroxonium ion H3O+ in bulk H3PW12O40 and of hydroxyl type species-in:the porous cesium salt Cs1.9H1.1PW12O40 After dehydration at a higher temperature, 573 K, the INS spectra showed the presence of hydroxyl groups in both samples, These four techniques provided a detailed description of the acidic features (nature, strength and number of the acid sites) of H3PW12O40 and Cs1.9H1.1PW12O40 samples in relation with their structure:ana hydration state.