A new solid high-proton conductor decatungstomolybdovanadogermanic. heteropoly acid (HPA) H(5)GeW(10-)MoVO(40)(.)21H(2)O has been synthesized for the first time by stepwise acidification and stepwise addition of solutions of the component elements. The product was characterized by chemical analysis, potentiometric titration, IR, UV, XRD and TG-DTA. The IR, UV and XRD indicate that H(5)GeW(10)MoVO(40)(.)21H(2)O possesses the Keggin structure. The TG-DTA curve shows the sequence of water loss in the acid, the amount of the loss, as well as the thermostability. The results of AC impedance measurement show that its proton conductivity is 3.58 x 10(-4) S cm(-1) at 18 degreesC and the activation energy for proton conduction is 31.82 kJ/mol. (C) 2004 Elsevier Ltd. All rights reserved.