The crystal structures of ANbOP(2)O(7) (A = Rb, Tl) compounds (space group P2(1/n)) were refined, showing three dimensional [NbOP2O7-]n frameworks more or less closely related to that of KNbOP2O7. The Tl one is significantly distorted. These frameworks, built up from NbO6 octahedra and P2O7 diphosphate groups sharing corners form large tunnels wherein the A(+) ions are situated. A discussion of all structures, including the bidimensional CsNbOP2O7 one, presents evolutions related to the size and polarizability of the A(+) ion.