Ionic conductivity and structure of crystalline, glassy Na4NbP3O12 and crystalline Na6Nb2P6O23 were studied by ac complex impedance and FTIR. FTIR spectra indicate the presence of pyrophosphate (P2O74-) groups in both crystalline and glassy Na4NbP3O12. The ionic conductivity of Na4NbP3O12 glass (sigma400-degrees-C = 3.1 X 10(-3) S cm-1) is at least two orders of magnitude higher than that of the crystalline phase (sigma400-degrees-C = 3.0 X 10(-5) S cm-1) due to large differences in the structure of the two modifications and the random distribution of Na+ ions over the equipotential sites in the glass network.