Research into fuel cell technology has increased dramatically recently due to the increasing gas prices and stricter pollution regulations. The need for a high proton conducting and thermally stable electrolyte for intermediate temperature (~100-300 °C) fuel cells has become a critical problem for proton exchange membrane (PEM) fuel cells in order to replace the present automobile internal combustion engines. Nafion is currently a very popular polymer electrolyte material with a proton conductivity of ~10⁻⁰̇·⁷⁷ ([Omega]cm)⁻¹ at 80 °C under heavily hydrated conditions. At temperatures above ~80 °C, Nafion dries out and its proton conductivity decreases to essentially zero. Research at ISU into alternative ceramic membranes for PEMs resulted in the development of alkali thio-hydroxogermanates (M[subscript x]GeS[subscript x](OH)₄[subscript x]·yH₂O where M=Na, K, Rb, Cs) with high proton conductivities of ~10⁻² ([Omega]cm)⁻¹ at 120 °C and thermal stabilities up to ~300 °C. In an effort to further increase the proton conductivity, especially at temperatures near room temperature and to increase the long term thermal stability, new ternary alkali thio-hydroxosilicogermanates where Si⁴ has been partially substituted for Ge⁴⁺ to form compositions of the form Rb[subscript z]Ge[subscript x]Si[subscript 1-x]S[subscript z](OH)[subscripr 4-z]·yH₂O, where z=2 and 3, and 0.5[less than or equal to]x[less than or equal to]1.0, have been prepared and characterized for the first time. It was found that the Ge-based phases were observed to have higher intermediate temperature proton conductivities than the Ge/Si phases. Studies were conducted on the proton conductivity as a function of the relative humidity and it was found that even low levels of hydration (~6% R.H.) greatly improved the proton conductivity at low temperatures indicating the important role of water in the proton conduction mechanism.