Exothermic reversible reactions are industrially very significant. Previous work has looked at finding the Attainable Region for elementary kinetics and in this paper we extend the work to find optimal reactors for more complex kinetics. Using these results we show how the complexity of the kinetics affects the nature of the optimal reactor. In particular it is found that the optimal reactor structure includes a novel element, namely two parallel structures, thr first of which has preheating followed by the following reactors connected in series : a plug flow reactor, CSTR, plug flow reactor and finally differential side stream reactor, while the second structure is a plug flow reactor. The side streams in the DSR are taken from various points along the plug flow reactor making up the second parallel structure. Furthermore, it is also shown that in order to satisfy tangency conditions on the boundary of the Attainable Region, the rate of reaction of the material in the side stream to the DSR and that in the DSR at the point where the side stream is added must be equal. The equations describing the DSR operation as well as the conditions describing the operation of the other reactors in the optimal reactor structure are given.