The simultaneous liquid-phase etherification of isobutene and isoamylenes with ethanol has been carried out at 353 K over Amberlyst (TM) 35. The initial molar ratios of alcohol/olefins and isobutene/isoamylenes were both maintained constant at 0.5 and 1, respectively. The effect of water presence on the reaction system has been studied by the addition of water to the initial reactant mixture, ranging from 0 to 1 wt.%. Under experimental conditions, product distribution, as well as etherification reaction rates, have been clearly affected by the addition of water. Water favors the formation of tertiary alcohols through olefins hydration because of its preferential adsorption on the active sites, while it hinders etherification and dimerization reactions. The inhibitory effect of water was more noticeable on dimerization than on etherification. Based on specific theories about the dissociation of the active sites from the catalytic network, the amount of water required to dissociate all the sulfonic groups has been estimated. Finally, the feasibility of the bioethanol use as a reactant has also been evaluated. (C) 2015 Elsevier B.V. All rights reserved.