The behavior of continuously operated, isothermal, fixed-bed reactors with heterogeneous catalytic reactions can be affected by enforced periodic variations of process parameters, e.g., feed concn., reactant flow rate, temp., pressure. The operation can lead to improved performance of a catalytic reactor by increasing the effective reaction rate or by changing the product distribution. In isothermal reactors the difference between steady-state and periodic processing are caused by a disturbance of the adsorption/desorption equil. at the catalytic surface and a change of the activity of the catalyst, a nonlinear dependence of the reaction rate on the parameter periodically varied, and an operation in the hysteresis interval of unstable reactors. Exptl. and theor. studies are described of a simple irreversible reaction, with the isothermal hydrogenation of C2H4 on a Pt-Al2O3 catalyst as the model. Compared with steady-state processing, the degree of conversion can be increased considerably. Because of the kinetics of the model reaction, the isothermal reactor was unstable in defined regimes of temps. and concns. Unsteady processing can be used to operate in the hysteresis interval and therefore in a region not obtainable in the steady state. [on SciFinder (R)]