This paper presents an explanation of why methyl tert-butyl ether (MTBE) production by reactive distillation may yield multiple solutions. Widely different composition profiles and conversions may, as already reported by Krishna and others, result with identical column specifications, depending on the initial estimates provided and the VLE models being used. A hypothesis yielding a qualitative understanding of this phenomenon has been developed. The inert n-butene plays a key role in the proposed explanation. As the reaction mixture is diluted with n-butene, the activity coefficient of methanol increases substantially and the temperature decreases. This dilution has a profound effect on the equilibrium conversion, enabling MTBE to escape from the reactive zone without decomposition. The minimum boiling point azeotrope between MTBE and methanol plays an important role in the internal transport mechanism for the heavy methanol, in particular when fed below or in the lower part of the reactive section.