Ethyl tert-butyl ether (ETBE) and isobutene are the only significant products of the sulfuric acid (less than or equal to 0.001 M) catalyzed reactions of tert-butyl alcohol in water/ethanol mixtures at 3 MPa and 170 degrees C. Equilibrium is established after a few minutes or less. A 10 parameter kinetic model which embodies a first order elimination reaction and a first order nucleophilic substitution reaction adequately describes the influences of reactant (ethanol, water, and tert-butyl alcohol) concentrations, acid concentration, and residence time on product yields. The fit of the model to the data improves when the influence of water on the solvent’s dielectric constant is included by the addition of two more parameters. One finding of the simulation effort is that protonated isobutene (the key ingredient in ETBE formation) forms only from tert-butyl alcohol (not isobutene) under the conditions employed in this work. Thus tert-butyl alcohol should be preferred over isobutene as a reactant for ETBE synthesis at elevated pressures and temperatures.