The NASICON-type Li1-xTi2-xNbx(PO4)(3) (0 <= x <= 0.5) has been prepared using a conventional solid-state reaction and characterized by powder X-ray (XRD), scanning electron microscopy (SEM), nuclear magnetic resonance (NMR) and impedance spectroscopy (IS). The substitution of Ti4+ by Nb5+ in LiTi2(PO4)(3) increases unit cell parameters of NASICON phases (S.G. R (3) over barc). The parallel XRD and NMR analysis confirmed the formation of secondary phases besides NASICON compounds. The NMR components detected in Nb-93 and P-31, MAS-NMR spectra have been ascribed to formation of crystalline NbPO5 and NASICON phases. The substitution of Ti4+ by Nb5+ produces detection of three equally spaced P-31 MAS-NMR P(OTi)(4-n)(ONb)(n) components, whose intensity changes with the Nb content of the NASICON phase. On the other hand, the broad P-31 NMR band detected at similar to -130 ppm has been ascribed to the formation of an amorphous Nb phosphate that strongly affects conductivity of samples. At 355 K, total conductivity decreased from 1.2 x 10(-7) S cm(-1) (Ea = 0.49 eV) to 6.0 x 10(-9) S cm(-1) (Ea = 0.56 eV) when going from LiTi2(PO4)(3) to Li0.5Ti1.5Nb0.5(PO4)(3). This important decrease has been related to the increment of activation energy (stronger Li-network interaction); however, further studies are required to understand the deleterious effect of niobium in Li1-xTi2-xNbx (PO4)(3) series.