HPF6 is said to form a hexahydrate of a unique clathrate structure in which [Formula: see text] ions occupy truncated-octahedral cages in a framework of hydrogen-bonded water molecules. N.m.r. studies show, however, that appreciable quantities of HF are incorporated in the lattice and that the composition is more properly [Formula: see text], a result supported by chemical analysis. H3O+ and HF appear to be distributed over the lattice sites previously assigned to water molecules so as to involve all H atoms in the hydrogen bonding. The 1H resonance line is narrowed by rapid proton diffusion down to 150 °K. The 19F line of encaged [Formula: see text] is narrowed by rotation above 75 °K and reaches the rigid-lattice shape only below 25 °K.