A high-speed pulse reactor setup for the study of chemical reactions which are catalyzed by a fixed bed is described. The benefits of this method compared to stationary-state measurements are demonstrated using the example of ethylene epoxidation on elemental silver. It is shown that the intrinsic catalyst reactivity depends strongly on oxygen loading and has a pronounced maximum for a loading of about 50 %. In contrast, the selectivity is independent thereof. The relevance of the method for research in catalysis and its relation to the well-established temporal analysis of products approach are illustrated and discussed.