Catalyst performance for the reaction of methanol and isobutylene (IB) to form methyl tert-butyl ether can be assessed based on maximum conversion. The equilibrium conversion attainable for this reaction is of practical interest since separation of products from reactants downstream of the reactor can be simplified as the extent of conversion increases. A framework is presented by which different catalysts can be compared on an internally consistent basis for different temperatures, isobutylene concentrations, and/or methanol/isobutylene mole ratios. An equilibrium expression which accounts for the presence of nonreacting components is presented to correlate the effect of methanol/IB ratios and IB concentrations with observed conversions for different catalysts. Assumptions inherent in this method are discussed and data for several types of ion-exchange catalysts illustrate use of this framework.