At 298 K cation conducting mixed antimonate Na0.8Ni0.6Sb0.4O2 exhibits an O3-type structure, with sodium ions in a distorted octahedral coordination, space group R-3m, a= 3.0496(1)angstrom, c=16.4060(4)angstrom. Na-23 NMR at 197-288 K reveals two main types of sodium atoms in the structure, ascribed to 18g and 3b positions. The temperature increase results in the sites averaging and NMR lines narrowing. The activation energy for the motion of sodium cations within one site estimated from Na-23 NMR data is ca. 1.0 kJ/mol at 215-288 K, much lower than the activation energy of both sodium hopping between the sites in a crystallite (ca. 22 kJ/mol at 330-370 K) and long-range transfer in ceramic material (ca. 48 kJ/mol at 294-775 K). (c) 2007 Elsevier B.V All rights reserved.