H-2 NMR has been used to reexamine-and clarify the phase behavior and molecular motions in solid D3O+ClO4- In the low-temperature phase reorientation about the molecular 3-fold axis of the D3O+ ion is confirmed. The NMR results above 200 K, together with calorimetric data, show some dramatic effects due to eutectic behavior arising from small quantities of water in excess of the monohydrate stoichiometry. The true phases of D3O+ClO4(-) have now been identified; only one solid-solid phase transition occurs, with hysteresis, at 245 K on cooling and 254 K on warming. Motion in the room temperature phase is not isotropic. Isotropic lines reported earlier presumably were due to the molten eutectic mixture.