The effect of water vapour in the feed on the synthesis of ethyl-tert-butyl ether (ETBE) produced from isobutene and ethanol was investigated. The kinetics of gas phase etherification and accompanying formation of tert-butyl alcohol (TBA) by Wells-Dawson acid heteropolyacid H6P2W18O62 as catalyst was determined at 40 degrees C. Rate expression in the form of a power equation was developed to correlate the experimental data over a wide range of reactants pressures. Water presence in the catalytic reactor inhibited the formation of ETBE and was introduced into the kinetic equation as an independent additional factor. The experimental evidence of ethanol and water sorption into the bulk of catalyst was confirmed by independent gas phase sorption experiments. The results of catalytic experiments were interpreted qualitatively on the basis of the model of catalytic system containing solid heteropolyacid in which tertiary ethers are synthesized. (C) 2007 Elsevier B.V. All rights reserved.