We study the ionic conductivity versus temperature and frequency of large Na2S single crystals by using a calibrated impedance apparatus. The experimental setup used for the ionic conductivity measurements up to 1350 K and its calibration are described. The apparatus allows to measure complex impedances between 0.1 Ω and 10 GΩ. The high temperature conductivity data were analyzed in terms of the conventional Frenkel defect model. We assume that cation vacancies and cation interstitials are the dominant intrinsic defects. The energy of motion was found to be 0.61 ± 0.05 eV for a cation vacancy. The energy of formation of a Frenkel defect pair is 2.51 ± 0.05 eV. Results are given that show clear evidence of a superionic behaviour close to the melting point, similar to the one found in alkaline earth fluorides and several halides. Furthermore, X-ray diffraction experiments on a high optical quality single crystal were performed. The cell parameter and the population parameter of Na+ were accurately determined (6.5373 Å and 0.988, respectively).