For the conceptual design of continuous countercurrent separative reactors, there is a need to predict the composition of the feasible top and bottom products. These products can be identified as stable singular points of the rectifying and stripping sections at infinite column height. General singular point equations are derived for the top and bottom products of separative reactors at countercurrent operation. They can be applied to phase equilibrium-controlled processes as well as to mass-transfer-controlled processes. For a given reactive mixture, all singular points at the top and at the bottom of a column are located on a unique potential singular point surface. It is demonstrated that interfacial vapour–liquid mass transfer has a strong impact on the feasible reactor products. Reactive distillation and reactive pervaporation of selected ideal and real reaction systems are used as illustrative examples.