The attainable region approach for reaction – mixing systems is combined with geometric methods for the feasibility of separations. The result is a systematic approach to identify the feasible compositions that can be achieved in processes combining simultaneous reaction, mixing, and separation. An activity-based formulation for both nonideal VLE and reaction-rate expressions is applied to develop hybrid reactor–separator models for the multiphase CSTR and PFR with simultaneous vapor removal. A reaction–separation vector is defined that satisfies the same geometric properties as the reaction vector. Therefore, the attainable region can be constructed following the existing procedure for reaction–mixing systems. This approach provides a method to generate feasible process alternatives. The technique is demonstrated on two nonideal ternary mixtures: the production of dimethyl ether by dehydration of methanol, and the production of methyl tert-butyl ether from isobutene and methanol. It is shown that for hybrid reaction–separation devices the entire composition space is not always attainable. In such cases, combining a hybrid device with traditional nonreactive separation is required to attain certain products.