A series of partly substituted cesium salts Cs1H6-1P2W18O62 of Wells-Dawson heteropolyacid has been prepared. The samples were catalytically active at temperatures as low as 40 degrees C despite low specific surface area of the order of 1 m(2) g(-1). Activity in gas phase synthesis of ethyl-tert-butyl ether related to mass of non-neutralized acid reached maximum at x = 5 and was higher than that of unsupported crystalline heteropolyacid. (c) 2006 Elsevier B.V. All rights reserved.