The title compound, tetrasodium nonamanganese octadecaoxide, Na(4.32)Mn(9)O(18), was synthesized by reacting Mn(2)O(3) with NaCl. One Mn atom occupies a site of 2/m symmetry, while all other atoms sit on mirror planes. The compound is isostructural with Na(4)Ti(4)Mn(5)O(18) and suggestive of Mn(3+)/Mn(4+) charge ordering. It has a double-tunnel structure built up from double and triple chains of MnO(6) octahedra and single chains of MnO(5) square pyramids by corner sharing. Disordered Na(+) cations occupy four crystallographic sites within the tunnels, including an unexpected new Na(+) site discovered inside the large S-shaped tunnel. A local-ordering model is used to show the possible Na(+) distribution, and the unit-cell evolution during charging/discharging is explained on the basis of this local-ordering model.