Differential scanning calorimetry on (Li1_J(K (.)2S 04 mixed crystals, where x = 0.01, 0.1, 0.3 and 0.5, revealed that, as the potassium content increases, the first high temperature phase of the intermediate LiKS04 phase at T -432 °C grows and shifts to higher temperature, and a double-phase mixture con sisting of LiKS04 and Li2S 04 exists for x< 0.5. Ionic conductivity measurements on (Li|_xK;t)2S 04 mixed crystals for x < 0.5 did show that the electrical conductivity increases as the Li2S 04 concentra tion increases, with an average activation energy of 0.9 eV. The enhanced electrical conductivity is pri marily a result of the increase in the Li+ ion (charge carriers) concentration and/or the formation of a diffuse space charge layer at the interface between the two phases.