Abstract Na 3MgZr(PO 4) 3 and Na 3MnZr(PO 4) 3 exist in the NASICON structure (space group R 3 ̄ c — D 6 3d ) with the lattice constants a 0 = (890.7 ± 0.6) pm, c 0 = (2222 ± 4) pm or a 0 = 900.0 pm, c 0 = 2239.3 pm, respectively. Intensity calculations of X-ray powder diagrams based on different models and 31P-MAS-NMR measurements indicate random distribution of Zr IV and Mg II or Mn II on the octahedral sites. The Na + ions are suggested to be equally distributed one on the Na I site and three on the Na II sites of the lattice. The compounds show incongruent melting at 1075 °C or 1025 °C, respectively. The conductivity of the manganese compound is about three times higher than for the magnesium compound. However, in comparison with Na 3Zr 2(SiO 4) 2(PO 4) the Na + ion conductivity at 300 °C is about three orders of magnitude lower, which is assumed to be caused by the highly irregular structure of the compounds.