A calibrated quadrupole mass spectrometer (QMS) is combined with a home-made positioning unit deriving from scanning electrochemical microscopy (SECM) to sequentially address 25 catalysts, which are placed in wells. Highly reproducible catalytic data are obtained under stagnant-point flow conditions by means of coupled gas feed and QMS capillaries. The reaction array can be heated and is fully seated from the atmosphere by a covering glass lid. Additionally, the reaction chamber is flushed with argon making it possible to study the structure-insensitive hydrogenation of ethene over SiO2-supported palladium catalysts unimpaired by oxygen poisoning. Since the derived semi-quantitative degree of conversion is proportional to the Pd metal area under fixed reaction conditions, spatially resolved mass spectrometry provides a novel method for rapidly estimating the metal surface areas of a Pd catalyst library. (c) 2004 Elsevier B.V. All rights reserved.