The effect of MgO and CaO addition to NH4NbWO6 type proton conductive solid electrolytes has been investigated. The NH4NbWO6 electrolyte has protonic conduction in moderate temperature. The test sample pellets were prepared by exchanging Rb+ to NH4+ in sintered RbNbWO6 by immersing into molten NH4NO3. Addition of a small amount of the MgO, CaO resulted in an increase of the thermal stability of the electrolyte.