This paper presents a method to determine the product composition regions-of single-feed reactive distillation columns. A new set of transformed compositions is introduced, and the resulting residue curve maps are studied for two reacting mixtures containing inerts. Due to the geometrical properties of the new composition variables, by using the residue curves it is possible to determine the feasibility of a desired separation from the extreme limiting operating conditions. As in conventional distillation, the generalization of the feasibility criterion gives a;:method to obtain the complete product composition region attainable in a single-feed reactive distillation column for a given quaternary feed. These concepts are applied to azeotrope-forming mixtures. An interesting behavior is observed for the highly nonideal reacting mixture isobutene-methanol-MTBE-butane. The minimum reflux ratio, for certain splits, corresponds to a finite number of trays in both column sections. The conditions for the occurrence of such a ease are given.