Structural sites occupied by lithium in the rhombohedral LiTi2-xZrx(PO4)(3) series (0 <= x <= 2) have been investigated by Li-7 NMR spectroscopy. At room temperature, the XRD patterns of the end-members of the series display rhombohedral R (3) over barc symmetry in LiTi2(PO4)(3) and triclinic C (l) over bar in LiZr2(PO4)(3). In the first compound, Li ions occupy M1 sites; however, in the second one Li occupy intermediate M-1/2 sites. As the temperature increases, a first-order displacive transformation is detected in the triclinic phase, but a second-order/disorder transition is detected in the rhombohedral phase. From the temperature dependence of the Li-7 NMR quadrupole constant (C-Q) of the two compounds, the evolution of M-1 and M-1/2 sites occupancy in the Nasicon conduction network has been deduced. At high temperatures, analyzed phases tend toward a disordered rhombohedral phase, in which both M-1 and M-1/2 sites are equally populated and in which lithium mobility is favored by the existence of vacant M, sites. According to this study, this phase can also be obtained by substituting Ti by Zr in the LiTi2-xZrx(PO4)(3) series.