Ionic conductivities have been investigated for mixed hydrogen alkali thiogermanates possessing the adamantane-like Ge4S104- structural complex anion. Anhydrous compounds of the form HxM4-xGe4S10 (M=K and Rb) and HxCS4-xGe4S10.yH(2)S for 0less than or equal toxless than or equal to4 and 0less than or equal toyless than or equal to1 were synthesized using a novel preparation method involving reacting the respective alkali thiogermanate glass or adamantane-like crystalline phase with liquid hydrogen sulfide. The reaction products were characterized by mass, thermogravimetric analysis, and Raman and infrared spectroscopies. Conductivity values in the range of 10(-10) to 10(-5) S/cm were determined from a.c. impedance measurements of the protonated phases below a typical thermal decomposition temperature of similar to175 degreesC. The decrease in ionic conductivity above the decomposition temperature is consistent with the loss of mobile protons as hydrogen sulfide. Higher proton mobility in the mixed hydrogen alkali thiogermanates vs. the anhydrous thiogermanic acid H4Ge4S10 may be partially attributed to lower decomposition temperatures and increased hydrogen bonding. (C) 2004 Elsevier B.V. All rights reserved.