Abstract: Experiments proved that the etherification between methanol and iso-butene catalyzed by a macroporous sulfonic acid resin in producing methyl tertbutyj ether is a reversible reaction. At a certain temperature, there is a definite methanol/isobutene molal ratio (Rao), below which the etherification rate is second-order on isobutene concentration and negative first-order on rnethanol. When the ratio is higher than Rao, the reaction becomes first-order on isobutene concentration and zero-order on methanol. The dependence of Rao on temperature can be expressed as follows: The disether ification (reverse reaction) rate is first-order on MTBE concentration. The reaction rate equations may be given as when Ra is smaller than Rao, and when Ra is larger than Rao. The reaction is as taking place with the electrophilic mechanism. The solvated proton hypothesis suggested by other authors cannot explain the transition rates at methanol/isobutene ratio = Rao. A novel liquid-solid microreactor containing about 50 mg of catalyst was used in carrying out the experiments.