A stainless steel cell with beryllium windows has been used for carrying out in situ Mössbauer spectroscopy studies (with a vertical γ-ray beam) at pressures up to 6.8 MPa and temperatures up to 773 K. The downflow of gas through the powdered catalyst sample minimizes bulk gas phase concentration gradients in the cell above the catalyst layer, and this allows reaction kinetics measurements to be routinely analyzed. Incorporation of a prereactor and/or postreactor makes it possible to model longitudinal variations of catalyst states in a packed catalyst bed.