A differentially pumped pressure cell has been developed to enable in situ low-energy ion scattering (LEIS) analysis of catalysts during chemical reactions. The cell is fully compatible with an electrostatic analyzer and is, therefore, very well suited to study rough, highly dispersed catalysts. The pressure cell is a continuous flow cell (space velocity 1.2×103 s–1) and allows observation of dynamic surface reactions with various, perfectly mixed gases at fully controlled partial pressures and temperatures up to 800 K. The design decreases the pressure gap by three orders of magnitude. This offers ample opportunities such as the determination of specific adsorption sites, surface coverages of different species during reactions, and information on growth processes, poisoning or chemically induced segregation. As an example, an in situ LEIS study of the CO oxidation over Pt is presented. By combining in situ LEIS and quadrupole mass spectroscopy, both the surface and gas composition could be monitored during this reaction.