The crystal structure of Na3InCl6 was determined at 200 K by the single-crystal X-ray analysis. The crystal belongs to a trigonal 13 system in which an isolated InCl63- anion and two crystallographically nonequivalent sodium ions Na(1) and Na(2) are confirmed. At 450 K, however, the Rietveld analysis suggested that a new site was occupied by the sodium ion forming vacancies at the original positions. In accordance with this observation, the Na-23 NMR spectrum using a single crystal showed a coalescence phenomenon of the two crystallographically different peaks at around 400 K. These observations support the dynamical chemical exchange between Na(1) and Na(2) through newly occupied Na(3) site. Although the ionic conductivity for this compound was not so high (2 x 10(-5) S cm(-1) at 500 K), the conduction path for the sodium ions was able to be proposed clearly. (C) 2004 Elsevier B.V. All rights reserved.