With surging interest in high energy density solid state lithium rechargeable batteries and discoveries of high voltage cathode materials such as LiMn2O4 and LiCO2, it is imperative that useful solid electrolytes possess a voltage window of about 5 volts. These solid electrolytes after being assembled into a cell will be subjected to a dc bias potential equal to cell voltage. The ability of composite electrolytes to withstand the dc potential has been determined, discussed, and presented. The experimental data include linear sweep and cyclic voltammetry measurements. The data are analyzed in terms of dielectric breakdown, ionization, and electrodic reactions.