Catalytic formation of ethyl-tert-butyl ether (ETBE) by nucleophilic addition of ethanol to isobutene on Dawson-type heteropolyacid H(6)P(2)W(18)O(62)(.)nH(2)O (HP2W) has been studied in the temperature range of 35-80 degrees C. The yield of ETBE reached a maximum of 43% at 40 degrees C and then decreased with increasing temperature. Such behaviour is typical for reversible exothermic reactions approaching equilibrium and results in the decrease of the yield with temperature. At 40 C the activity of the catalyst depended on the pre-treatment temperature and, hence, on the content of water of crystallisation. Samples pre-heated at 80 and 95 degrees C, containing about eight H2O molecules per anion, exhibited the highest activity. Under these conditions the ETBE yield is slightly above 40%. The samples calcined at 150 and 200 degrees C, containing about three H2O per anion, exhibited a yield of about 27%. Parallel sorption experiments of ethanol on octahydrated and trihydrated HP2W have shown that, in the former case, the sorption of ethanol was nearly twice as large as that in the latter case. The differences show that the sorption capacity of the samples depends on the secondary structure of the heteropolyacid. These differences have been assumed also as explanation for the observed levels of catalytic activity. In all experiments carried out at 40 C the selectivity to ETBE was over 90% but it decreased to a few percent at 80 degrees C. (c) 2005 Elsevier B.V. All rights reserved.