In order to evaluate the activities of granular catalysts, a microreactor having six channels was fabricated using silicon and glass plates by means of photolithography and wet etching. Using anodic bonding, the plates were configured so as to be gas tight. The dimensions of the channels were I mm in width, 0.3 mm in depth and 6 rum in length. Catalysts for the oxidation of carbon monoxide in hydrogen, as examples, were prepared by impregnating Pt, Rh, Ru, Co, Ni, Cu and Pd on gamma-alumina support particles which were sieved to 150-210 mum, and 9-mg aliquots of the catalysts were packed in the channels. The experiments were carried out in the temperature range of 120-200degreesC, and the compositions in the products were determined by quadrupole mass spectrometry. The findings show that the microchannel reactor could be used for the efficient screening of granular catalysts.