According to X-ray investigations on single crystals the crystal structure of Na3O(CN) = (CN)ONa3 derives from the anti-perovskite type of structure (a = 454.3(1) pm, Pm3m); the CN- anion is disordered. Above 230-degrees-C, Na3O(CN) shows an exceptionally high sodium ion conductivity (10(-2) ohm-1 cm-1), which is about four orders of magnitude larger than that at room temperature. The increased conductivity of Na3O(CN) seems to result in part from a melting of the sodium sublattice and the rotational disorder of the CN- groups.