The influence of proton hydration in Wells-Dawson H6P2W18O62 (HP2W) and Keggin H3PW12O40 (HPW) type heteropolyacids on their catalytic activity in ethyl-tert-butyl ether (ETBE) synthesis in gas phase was investigated. In the case of samples with the content of crystallisation water per one proton below 0.8 ([H2O]/[H+] < 0.8), the catalytic activity related to the mass unit of the HP2W was higher than that of the HPW in accordance with the order of proton concentration in mass unit. On the other hand, the activity related to one mole of protons indicated higher activity in the HPW than in the HP2W in accordance with the order of acid strength. In the series of samples with [H2O]/[H+] > 0.8 the activity was much higher for the HP2W than the HPW and maximum of both catalytic activity and ethanol sorption capacity was observed for the HP2W. The possible role of the secondary structure of the hydrates was discussed.