The structure and dynamics of trimethylene oxide (TMO) and ethylene oxide (EO) structure I (sl) hydrates are reported from single-crystal X-ray diffraction and H-2 NMR spectroscopic measurements. The guest molecule positions in the large cage were determined with considerable improvement over previous diffraction work so that a dynamic model that was consistent with these orientations could be developed to explain the H-2 NMR data. Reorientations are shown to take place among both symmetry-related and symmetry-independent sites, 16 positions in all. Because of the prochiral nature of the molecules, both guests show H-2 NMR line shapes with large asymmetry parameters, rather unusual for guest molecules in the sI hydrate large cage. The results also show that the dipolar axis of the TMO molecule lies close to the 4 bar axis of the cage on average, whereas for EO, this is not the case. For TMO, progressive alignment of the polar axis with decrease of temperature then allows the dipoles to interact more strongly until dipole reversal is quenched at the ordering transition. The lack of ordering of EO is consistent with the much weaker alignment of the molecular dipoles along the 4 bar axis. With the new complementary information on the structure and dynamics from crystallography and NMR, it is possible to understand why the large cage guests order in the large cage of sl hydrate for TMO hydrate but not for EO hydrate.