For practical reasons, ATR-IR probes are usually mounted in the bottom of an autoclave, inside a small cavity. Using a transparent mock-up, we observed that it is very likely that catalyst particles settle inside this cavity, right above the crystal. The impeller speed needed to remove these deposited particles depends on the impeller dimensions and the density of the catalyst particles. Trends in this catalyst settling behaviour agreed with the Zwietcring' correlation indicating poor mixing characteristics due to the presence of the cavity. The presence of a layer of catalyst particles on the crystal leads to overestimation of the observed reaction rates measured by ATR-IR. Several indicative experiments of the hydrogenation of naphthalene to decalin over Pd-based catalysts, involving step changes, showed dynamic responses and trends in observed reaction rates that can be used to reveal these effects in closed autoclaves. A simple model that assumes an ideally mixed bulk phase and a stagnant layer of catalyst particles in the cavity describes the observed phenomena well. (c) 2007 Published by Elsevier B.V.