A series of NASICON type compounds A(1-x)Zr(2-x)Nb(x)(PO4)(3) (A=H, Li; x=0, 0.02, 0.1, 0.2) have been studied by H-1, Li-7, P-31 NMR and impedance spectroscopy. Niobium substitution leads to cation mobility enhancement and favors the triclinic-rhombohedral phase transition. The phase transition and a local lithium motion are highly increased by vacancy formation, while cation conductivity is increased by interstitials formation in LiZr2(PO4)(3) structure. (C) 2004 Elsevier B.V. All rights reserved.