The low temperature dynamics of molecular H20 has been studied in gypsum (CaSQ" 2H20), bassanite (CaSO4 0.5 H20 ) and cordierite (Mg2A14SisO18-H20) using neutron spectroscopy. In gypsum, where the H20 is structurally bound and the protons can be located in diffraction studies, inter-and intramolecular motions of the H20 and SO4 groups have been observed. In bassanite, two broad bands in a spectrum recorded at 150 K imply that at this temperature the H20 is dynamically disordered and a static description of the room temperature structure is inappropriate. At low temperatures the H20 molecules order, and at 3 K the spectrum consists of a number of sharp bands. In synthetic, alkali-free hydrous cordierite the H20 molecules are dynamically disordered. This has been shown by earlier NMR and room temperature quasielastic neutron scattering studies, and has been confirmed here by low temperature (50 K) quasielastic neutron scattering. A temperature decrease down to 3 K induces only gradual changes in the inelastic spectrum, and the three bands which we have observed are significantly broadened. It is therefore concluded, that the dynamic disorder persists even down to 3 K. This is in agreement with results from NMR spetroscopic experiments for beryl. High resolution time-of-flight spectra obtained at 3 K exclude tunneling.