Double phosphates of Na8-2xM4+x2+(P2O7)(4) formulation (M = Mg, Mn, Co, Ni) have been isolated either in the crystalline state or in the glassy form and, for a given M2+ cation, glasses are of the same composition as the crystalline phase. Na+ ion conductivity in the glasses is discussed as a function of sodium content and optical basicity. The various electrical parameters determined for the glasses, where M = Mn, Co and Ni, are compared with those of the corresponding crystalline phases.