The catalytic activity of various strong acid ion-exchange resins on the synthesis of methyl tert-butyl ether (MtBE) from methanol and isobutene has been investigated. Relative to Amberlyst 15, Kastel CS 381 and Amberlyst CSP have similar rate constants, whereas Duolite ES 276 and Amberlyst XE 307 have significantly higher and Duolite C26 and Duolite C16P substantially lower rate constants. All resins show a great decrease in catalytic activity if part of the protons is exchanged by sodium ions. At 10% proton capacity the rate constants per equivalent acid are reduced by a factor of 9 (for Amberlyst XE 307 and Kastel CS 381) to more than a factor 20 for Amberlyst 15 and Duolite ES 276, resulting in 100-200 times lower MtBE production rates. Depending on the catalyst applied, mass transfer limitations start to occur between 50 and 80 degrees C. Values of the effective diffusion coefficient of isobutene varied between 0.4 x 10(-9) and 4.1 x 10(-9) m(2) s(-1) at 80 degrees C.