Proton magnetic resonance measurements have been made on pentamethylethanol hemihydrate and pentamethylethanol hemideuteriohydrate over a wide range of temperature. Line narrowing occurs between –200 and –150°C due to methyl group rotation and at –50°C due to a large-scale motion involving both the alkyl groups and the protons of the ice-like, hydrogen-bonded chain which links together the alcohol and water molecules. Theoretical second moments calculated from X-ray data are correlated with the experimental values by assuming that the alcohol molecules undergo reorientation about their C—O bonds in a manner which allows co-ordinated motion of the protons in the hydrogen-bonded system. The experimental second moments of pentamethylethanol hemideuteriohydrate support this hypothesis.