Abstract The structure of an end member (x=0 of the NASICON solid solution system Na 1+x+4yZr 2−ySi xP 3−xO 12, 0≦x≦3, 0≦y≦1 has been investigated at room temperature and at 993K. At room temperature the NASICON framework shows static disorder: interstitial sites inside the ZrO 6-coordination octahedron and split oxygen sites. We present a model which prompts that both effects are correlated. The occupancy of the Na2 site - not significant at room temperature - was refined to about 10% at 993K. The results from both measurements can be correlated only, if at room temperature Na + is located in the interstitial sites and if the transition of these ions to the Na2 site occurs at high temperature. Additional refinement of higher order temperature coefficients of the Na sites at 993K yielded a Na diffusion path Na1-Na2 similar to that found in other NASICON compositions. The height of the effective one particle potential barrier agrees quite well with the activation enthalpy measured on single crystals with impedance methods (part III). Therefore we conclude that in this NASICON composition the main diffusion process is a bulk process.