The title compound, (NH(4))(5)Na(7)[Mo(5)P(2)O(23)](2)·21H(2)O, was prepared under atmospheric conditions in aqueous solution at room temperature. The structure contains the [Mo(5)P(2)O(23)](6-) heteropolyoxometallate anion, which has been previously reported a number of times with a variety of differing counter-cations. Each anion is built up of five MoO(6) octa-hedra sharing an edge and forming a ring which is closed by common corners of the terminal octa-hedra. The rings are closed on both sides by two asymmetric PO(4) tetra-hedra, sharing three corners with three MoO(6) octa-hedra. The anions are chiral and the two independent anions in the asymmetric unit were arbitarily chosen with the same chirality, but the centrosymmetric crystal contains both enanti-omers. The structure can alternatively be described as a succession of layers parallel to (101), formed by the [Mo(5)P(2)O(23)](6-) anions and linked by sodium chains. Water mol-ecules and ammonium ions fill the remaining space and ensure the cohesion through extensive N-H⋯O and O-H⋯O hydrogen bonding.