The possibility of conversion increase by periodic control of chemical reactors is studied. Method of determination of parameters of the periodic input control (amplitude and period) is presented. As an optimality criterion the time integral of the difference of the conversion under periodic and constant operation of the reactor is chosen. The proposed method is of gradient type, the gradient components are computed with the help of the system of adjoint equations. The method is illustrated by the example of the following catalytic system : continuous stirred tank reactor, narrow active region catalyst and catalytic reaction described by Langmuir-Hinshelwood kinetics. In this example the improvement of the reactor performance is significant.