Control of the temperature rise across a reactor where a portion of the product stream flowing from the reactor is recycled and mixed with the feed flowing to the reactor is accomplished by controlling the flow rate of the recycle stream to thus control the concentration of the reactants flowing to the reactor in such a manner that a desired temperature differential across the reactor is obtained. The inlet temperature to the reactor is controlled by manipulating the temperature of the recycle stream to thus maintain the inlet temperature to the reactor at some desired temperature. Ratio control is utilized to maintain a desired ratio of reactants flowing to the reactor if more than one reactant is flowing to the reactor.