Various experimental studies on a new fast A(g+) ion-conducting composite electrolyte system : (1-x) (0.75Agl:0.25AgCl):xAl(2)O(3) are reported. Undried Al2O, particles of size < 10 mu m were used. The conventional matrix material Agl has been replaced by a new mixed 0.75Agl:0.25AgCl quenched and/or annealed host compound. Conductivity enhancements similar to 10 from the annealed host and similar to 3 times from the quenched host obtained for the composition 0.7(0.75Agl:0.25AgCl):0.3Al(2)O(3), can be explained on the basis of the space charge interface mechanism. Direct measurements of ionic mobility mu as a function of temperature together with the conductivity sigma were carried out for the best composition. Subsequently, the mobile ion concentration n values were calculated from mu and sigma data. The value of heat of ion transport q* obtained from the plot of thermoelectric power theta versus 1/T supports Rice and Roth’s free ion theory for superionic conductors. Using the best composition as an electrolyte various solid state batteries were fabricated and studied at room temperature with different cathode preparations and load conditions.