The simultaneous etherification of isobutene and isoamylenes with ethanol has been studied using macroreticular acid ion-exchange resins as catalyst. Most of the experiments were carried out over Amberlyst-35. In addition, Amberlyst-15 and Purolite CT-275 were also tested. Chemical equilibrium of four chemical reactions was studied: ethyl tert-butyl ether formation, tert-amyl ethyl ether formation from isoamylenes (2-methyl-1-butene and 2-methyl-2-butene) and isomerization reaction between both isoamylenes. Equilibrium data were obtained in a batchwise stirred tank reactor operated at 2.0 MPa and within the temperature range from 323 to 353 K. Experimental molar standard enthalpy and entropy changes of reaction were determined for each reaction. From these data, the molar enthalpy change of formation of ethyl tert-butyl ether and tert-amyl ethyl ether were estimated. Besides, the chemical equilibrium between both diisobutene dimers, 2,4,4-trimethyl-1-pentene and 2,4,4-trimethyl-2-pentene, was evaluated. A good agreement between thermodynamic results for the simultaneous etherification carried out in this work and those obtained for the isolated ethyl tert-butyl ether and tert-amyl ethyl ether systems was obtained. (C) 2013 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.