It has been hypothesized hitherto that the liquid-phase synthesis of MTBE on ion-exchange resins proceeds quasi-homogeneously in excess methanol by an ionic mechanism whose rate-limiting step is the protonation of the alkene, and quasi-heterogeneously at very low methanol content likely by a concerted proton transfer involving adsorbed reactants. However, recent knowledge of the reaction involving the effect of the non-ideality of the liquid phase on the kinetics of the reaction, and the inhibitor effect of MTBE on the reaction rate, in addition to new data on byproducts formation, suggest that MTBE synthesis can be looked at as a quasi-heterogeneous catalysis independently of the methanol content of the liquid phase. Moreover, a transition between a Eley-Rideal mechanism and an Langmuir-Hinshelwood one can be assumed as the methanol concentration of the liquid phase decreases.