The attainable region approach for reaction-mixing Systems was applied to Systems with simultaneous reaction and separation. A model developed for a countercurrent cascade of two-phase CSTRs can be considered as a surrogate for the stripping or rectifying sections of a reactive distillation column. An overall composition vector defined satisfies the same geometric properties as the reaction vector. The necessary conditions for the reaction-mixing attainable region were applied to the cascade of two-phase CSTRs. This article focuses only on the countercurrent cascade of two-phase CSTRs; therefore, the region identified is a feasible region, which is a subset of the attainable region for reaction-separation systems. For consecutive reactions, the boundary of the feasible region is given by the trajectory for a cocurrent cascade of two-phase CSTRs. The overall composition vector for the cocurrent cascade of two-phase CSTRs is tangent to the trajectory. For single reversible reactions, the attainable region for single-phase reactors is significantly expanded by using a countercurrent cascade of vapor-liquid CSTRs. Two examples used are transalkylation of toluene and the Synthesis of isopropyl acetate.