The dependence of phase formation, sintering behavior, and electrical characteristics of Sodium Superionic Conductor (NASICON) compounds on sintering temperature, time, and cooling process was investigated. In the von Alpen-type composition Na3.2Zr1.3Si2.2P0.8O10.5, ZrO2 second phase is in thermal equilibrium with crystalline NASICON and liquid phase above 1320 degrees C, and when cooled through 1260-1320 degrees C, the crystalline NASICON was formed by reaction between the ZrO2 second phase and the liquid phase. Maximum relative densities of 96 and 91% were obtained for compositions Na3Zr2Si2PO12 and Na3.2Zr1.3Si2.2P0.8O10.5, respectively. For these compositions, the maximum ionic conductivity and the minimum migration barrier height were 0.45 ohm(-1) cm(-1) and 0.07 eV, respectively. The migration barrier height of the high temperature form (space group: R3c) is about 30-40% of that of the low temperature form (space group: C2/c). Ionic conductivity increases with increasing sinterability, and a considerably large amount of glass phase in Na3.2Zr1.3Si2.2P0.8O10.5 ceramics significantly lowers ionic conductivity above the transition temperature.