Global reaction enhancement by forced composition cycling was investigated by calculation. In this study, a catalytic reaction mechanism proposed for NO reduction by hydrocarbons over alumina was employed. Langmuir- Hinshelwood kinetics was assumed in the mechanism. The characteristics of the mechanism were as follows: (1) two successive intermediates, (2) competitive consumption of the active second intermediate. An important finding was that global enhancement was attained by the forced cycling. Additionally, optimization of the periodic operation indicated that the maximum enhancement factor was about 1.7. The reason for the global reaction enhancement was discussed. In this system, the number of system condition functions, coverage on the catalyst surface by reactive species, is greater than that of the operating variables, concentrations of reactant gases. This makes the trajectory of the condition functions under the forced cycling frequently move apart from, or into the region where it is restricted at the steady state. This seems essential for the enhancement under unsteady conditions in the model.