The state of sodium inserted in the hard carbon electrode of a sodium ion battery having practical cyclability was investigated using solid state Na-23 NMR. The spectra of carbon samples charged (reduced) above 50 mAh g(-1) showed clear three components. Two peaks at 9.9 ppm and 5.2 ppm were ascribed to reversible sodium stored between disordered graphene sheets in hard carbon because the shift of the peaks was invariable with changing strength of external magnetic field. One broad signal at about -9 to -16 ppm was assigned to sodium in heterogeneously distributed closed nanopores in hard carbon. Low temperature Na-23 static and magic angle spinning NMR spectra didn't split or shift whereas the spectral pattern of Li-7 NMR for lithium-inserted hard carbon changes depending on the temperature. This strongly suggests that the exchange of sodium atoms between different sites in hard carbon is slow. These studies show that sodium doesn't form quasi-metallic clusters in closed nanopores of hard carbon although sodium assembles at nanopores while the cell is electrochemically charged. (C) 2012 Elsevier B.V. All rights reserved.