X-ray absorption spectroscopy measurements of catalytic reactions have been instrumental in advancing the understanding of catalytic processes. These measurements require an in situ catalysis reaction cell with unique properties. Here we describe the design and initial operation of an in situ/operando catalysis reaction cell for transmission X-ray absorption spectroscopy measurements. The cell is designed: to be an ideal catalytic reactor with no mass transfer effects; to give the same conversion and selectivity under similar space velocities as standard laboratory micro-reactors; to be operational temperatures up to 600 degrees C and pressures up to 14 bar; to be X-ray transparent allowing XAS measurement to be collected in transmission for all elements with Z >= 23 (vanadium K-edge at 5.5 keV); to measure the actual catalyst bed temperature; to not use o-ring seals, or water cooling; to be robust, compact, easy to assemble, and use, and relatively low cost to produce. The heart of the cell is fabricated from an X-ray transparent beryllium tube that forms a plug flow reactor. XAFS data recorded during the reduction of a Re/gamma-Al2O3 catalyst as a function of hydrogen pressure from 0.05 to 8 bar, and from a Pt-Sn/gamma-Al2O3 catalyst during n-heptane reforming are given as initial examples of the versatility of the reactor. (c) 2006 Elsevier B.V. All rights reserved.