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 separations is required to attain certain products.