Direct oxidation of propylene by oxygen to propylene oxide (PO) has been studied through the application of the techniques of combinatorial catalysis. Catalytic materials containing single and binary metal components were prepared by impregnating standard gamma-Al2O3 pellets. In the first stage, 34 single component catalytic materials at three different metal loading levels were prepared and screened for PO activity and selectivity using array channel microreactors and mass spectrometry. Experiments were conducted at a GSHV of 20,000 h(-1), 10 1 kPa pressure and over a temperature range of 200-350degreesC. Following a matrix inversion technique to deconvolute the mass spectrometric intensity measurements, signals that were directly attributable to PO were calculated. From these determinations, the elements Rh, Mn, and Mo were the most PO active single metal catalysts on gamma-Al2O3. For acetone (AT) Rh, Pb, and Ir were somewhat effective, while Cu, Mn, and W favored some acrolein (AL) formation. In the second step, catalytic materials containing binary combinations of metals were prepared using a variety of strategies. However, the binary catalytic materials that exhibited the highest PO production levels always contained Rh. The binary combinations that exhibited superior PO production levels were Rh-V, Rh-Cr, Rh-Sn, Rh-In, Rh-Mo, and Rh-Sm, albeit substantial CO2 formation. On the other hand, Rh-Ag, Rh-Zn, and Rh-Cr combinations were significant leads with regard to high PO and low CO2 production. These findings call for the undertaking of detailed secondary screening studies to confirm the primary screening results reported here and to obtain information on the durabilities of these catalytic materials. (C) 2003 Elsevier B.V. All rights reserved.