The various approaches to enhancing the conductivity of Na 2 SO 4 are reviewed. The role of the size of the dopant cation on the conductivity enhancement has been emphasized. As for anion doping, apart from the size, the role of the shape and orientational ordering of the dopant ion has been highlighted. The structure of the guest ion appears to in6uence the stabilization of Na 2 SO 4 +I at low temperatures. A recent development has been the formation of Na 2 SO 4 -based composites. In this work, the stabilization of the Na 2 SO 4 +I phase for the 4 m/o La 2 (SO 4 ) 3 composition in the Na 2 SO 4 +La 2 (SO 4 ) 3 system has been established at 1203C through structural evidence. Several new features appear in the conductivity behavior of the Na 2 SO 4 +Al 2 O 3 composite system. In contrast to a previous study, we now observe and report the formation of the high conducting Na -alumina phase for the 5 m/o Al 2 O 3 composition. As before, two peaks are observed in the conductivity+composition plot, a feature not commonly encountered in known composite systems. Unlike other composite electrolyte systems, the size of the dispersoid phase does not appear to a4ect the conductivity enhancement. Furthermore, in most known systems, -Al 2 O 3 is used as a dispersoid. However, enhancements in conductivity for the Na 2 SO 4 +Al 2 O 3 system have been observed only when the identi-5ed phase is -Al 2 O 3 .