In order to industrialize NaSICON materials, modern fabrication techniques have to be used and one of those techniques for producing large-scale electrolyte sheets with 10-300 mu m thickness is tape casting. Such technique however requires a sintering step at high temperatures leading to sodium depletion due to evaporation. The sodium loss becomes more significant for large-area and thin components. In order to investigate and compensate the sodium loss, NaSICON compositions with sodium excess were prepared, i.e. Na3Zr2Si2PO12 + x Na2O (0 <= x <= 0.2). The sodium loss can be reduced by applying a two-step sintering process (1250 degrees C for only 0.5 h and then at 1230 degrees C for 5 h). Several characterization techniques were used to analyze the resulting ceramics, the sodium depletion and its consequence on electrical conductivity. Chemical analyses indicated that all compositions were sodium deficient. Furthermore, the weight loss was investigated by thermogravimetric analysis confirming the reduction of weight loss by a factor 2 by applying a two-step sintering procedure with lower second sintering temperature. Initial thermodynamic calculations of the phase equilibria at high temperatures confirm the predominant evaporation of sodium. The highest electrical conductivity (1.6 10(-3) S cm(-1) at 25 degrees C) was measured for the composition showing the least sodium deficiency (x = 0.2). Furthermore, the activation energy of bulk and grain boundary conductivity decreased with increasing x in system.