In the search for novel sodium-ion conductors to be used in batteries for grid application, the thoroughly studied class of NASICON materials is of great interest due to compositional diversity and high ionic conductivity. The solid solution Na3 + Sc-x(2)(SiO4)(x)(PO4)(3) (-) (x) with 0.05 <= x <= 0.8 was investigated for the first time. The various compositions were synthesized by solid state reaction and their crystallographic and electrical properties were measured. As a result, one of the best sodium-conductive NASICON materials to date was obtained for x = 0.4 (sigma(Na,Total) = 6.9 x 10(-4) S cm(-1) at 25 degrees C). Furthermore, the importance of the sodium concentration and size of lattice parameters on the ionic conductivity were investigated. The bulk ionic conductivity was correlated with the structural parameters along the conduction pathway of the sodium ions and confirm the key influence of the interatomic Na-O distances on sodium ion transport. (C) 2016 Elsevier B.V. All rights reserved.