Cs-containing salt or solid acid (CS2CO3 or CsHSO4) and dodecatungstophosphoric acid (H(3)PW(12)O(40)(.)nH(2)O:WPA-nH(2)O) were mechanically milled aiming at synthesis of chemically durable WPA salts and their composites. In the CS2CO3-WPA system, XRD patterns suggested the formation of partially substituted CsxH3-xPW12O40 after milling. After the formation of CsxH3-xPW12O40, chemical durability of the CS2CO3-WPA composites markedly improved. The proton conductivity of the CS2CO3-WPA composites containing CsxH3-xPW12O40 increased with an increase in the WPA content. CsxH3-xPW12O40 obtained from 40CS(2)CO(3)(.)60(WPA(.)6H(2)O) by milling showed 2 x 10(-2) S cm(-1) at 60 degrees C and 90% relative humidity (R.H.), while the conductivity drastically decreased under a dry N-2 atmosphere. In the CsHSO4-NATA system, the formation of CsxH3-xPW12O40 was not confirmed from XRD patterns after mechanical milling, whereas the water solubility of the CsHSO4-WPA composites decreased. The conductivity of 95CsHSO(4)(.)5(WPA(.)6H(2)O) with milling increased with increasing humidity and became 1 x 10(-2) S cm(-1) at 60 degrees C and 90% R.H. The mechanically milled 95CsHSO(4)(.)5(WPA(.)6H(2)O) composite exhibited a steep increase in conductivity at around 130 degrees C on heating even in a dry N, atmosphere and the conductivity reached about 10(-3) S cm(-1) at 150 degrees C, which should be ascribed to the superprotonic phase transition of CsHSO4 in the composite. (c) 2007 Elsevier B.V. All rights reserved.