Abstract Time of flight neutron powder data were collected at elevated temperatures on a sample of NASICON of composition Na 3.17Zr 1.93Si 1.9P 1.1O 12. This sample, prepared by reheating a commerically available NASICON to minimize the ZrO 2 impurity, has been previously structurally refined from room temperature neutron diffraction data collected at Argonne National Laboratory. Conductivity measurements show a cusp at ca. 170°C attributed to the monoclinic-rhombohedral transition and the purpose of this work was to explore the nature of the transition. Data runs were made at 143, 173, 202, 300, and 391°C. No evidence for a crystal system change was found. The highest temperature data sets could not be refined satisfactorily by the Rietveld method using a rhombohedral model, but rather required a monoclinic cell in space group C2 c to achieve convergence. An explanation for the conductivity cusp is given in terms of a disordering of the sodium ions and a change in conduction pathways which lowers the barriers to ion movement.