The structure of completely exchanged Na+/Pr3+-beta "-alumina crystals have been investigated by single crystal X-ray diffraction methods at room temperature. Ionic distributions within the conduction planes are correlated with the crystal composition. Up to 52% of the lanthanide ions are found to occupy BR positions. The conduction behaviour of Na+/Pr3+-beta "-Al2O3 crystals with varying degree of exchange were determined by impedance spectroscopy in a temperature range of 300-1000 K. With growing Pr3+ content the ionic conductivity (sigma) is lowered connected with an increasing activation energy E(a). For a given degree of exchange sigma scatters of about 1-2 orders of magnitude and E(a) within 0.15-0.2 eV. These deviations can be understood by the different crystal compositions influencing the thickness of the conduction slabs and the ionic distributions.