The phase equilibria and conductivities of the LiF-LiH, LiF-LiOH, LiF-Li/sub 2/O, Li/sub 2/S-Li/sub 2/O, Li/sub 2/S-LiCl and Li/sub 2/S-LiBr systems were investigated. All ternary single phases and two-phase mixtures are solid electrolytes which are thermodynamically stable in respect of reaction with elemental lithium (anode) and at practically useful, low lithium activities (cathode). The conductivity normally increases with decreasing thermodynamic stability and vice versa. The conductivity may be optimized in the case of solid solutions by selecting a composition with a decomposition voltage just above the value required by the cathode material employed. All materials are isotropic in structure and no dendrite formation was observed. This allows their use in rechargeable, thin film electrolyte batteries.