A quaternary catalyst library of 56 samples comprising all combinations of four elements,viz. Ag, Co, Cu. In. with six equally spaced atomic fraction increments from 0 to 1 was prepared by impregnation of a proprietary mesoporous alumina support. Catalytic properties of the library were tested in the selective catalytic reduction (SCR) of NOx by propane under lean conditions in the temperature range 400-500 degreesC. The catalytic data acquired by a parallel 64-channel microreactor system with automated time-of-flight mass spectrometric analysis have been evaluated regarding selectivity-compositional relationships, synergistic effects for NOx conversion, and efficiency of propane utilization. Full conversion of NOx is achieved over Ag-Co combinations at 450 degreesC with N-2 selectivities of more than 90% and reductant utilization of 20% in a feed of 1500 ppm NO, 1500 ppm propane and 5 vol.% O-2 (space velocity of 36,000 cm(3) g(cat)(-1) h(-1)). For the single-component catalysts Ag/Al2O3, Co/Al2O3, Cu/Al2O3, and In/Al2O3, the state of the elements on the mesoporous alumina was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). Cobalt forms a spinel-like cobalt aluminate phase whereas copper and indium are present as oxides with small sizes not detectable by XRD. Silver occurs in both metallic state and as Ag2O. and forms Ag-n clusters of at least two different sizes, predominantly with diameters of about 30 nm. The conclusions are consistent with the reducibility of the single-component catalysts samples by H-2. Surface area measurements and pore size distributions revealed reasonable modifications of the textural properties. The main pore size of the alumina support is decreased from 7 to ca. 5 nm after loading of the active components.