Selective catalytic reduction of NO by C3H6 was investigated using the tools of combinatorial chemistry under both stoichiometric and fuel-lean conditions. Fifty six quaternary Pt-Pd-In-Na combination catalysts were prepared by impregnating gamma -Al2O3 pellets with precursor solution mixtures prepared automatically using a micro-jet liquid dispensation system. Performances of the catalysts were evaluated in array microreactors using mass spectrometry in the temperature range 200-550 degreesC. A number of multi-metallic combinations showed excellent NO reduction activities under stoichiometric conditions and over a broad temperature range. However, the performance of the entire library under fuel-lean conditions was poor.