Nominal Na1.4Al0.4M1.6(PO4)(3) (M = Ge, Sn, Ti, Hf, Zr) NASICON compounds have been prepared as polycrystalline powders. Rietveld analysis of X-ray and neutron diffraction patterns showed that M = Ge and So samples crystallize in the R space group (s.g.) and that M = Ti, Hf, Zr compositions crystallize in the R c s.g. Data analyses indicated that aluminum contents into the main NASICON phases are always smaller than the nominal ones. Al-27, P-31 and Na-23 magic angle spinning-nuclear magnetic resonance (MAS-NMR) spectroscopies have been used to deduce aluminum contents and local atomic environments. In samples with high aluminum contents, M = Ge, Sn and Ti, electrical conductivity has been deduced from impedance spectroscopy data. The titanium sample incorporates the maximum aluminum content and displays the lowest activation energy, 0.52(1) eV, and the highest conductivity, sigma(T)(400 K)= 1.5(3) x 10(-5) S cm(-1). The influence of structural parameters on conductivity values has been analyzed with the bond valence sum method. With this technique, the most probable conduction pathways in Na1+xAlxTi2-x(PO4)(3) and Na1+xInxZr2-x(PO4)(3) samples have also been deduced. (c) 2005 Elsevier B.V. All rights reserved.