The selectivities of iridium (Ir), platinum (Pt) and rhodium (Rh) catalysts for reducing NO in the presence of excess O2 in two synthetic fuel-lean exhaust gases have been determined, before and after a severe heat treatment. When tested at 500°C, the order of selectivity was Ir>>Rh>Pt for fresh catalysts, and Pt>Rh for treated catalysts. When tested at 400°C, the treated Pt and Rh catalysts were identical in selectivity to the fresh Ir catalyst. A catalyst called Leanox* was developed which showed higher NO selectivity than the fresh Ir catalyst. The advantages of incorporating this catalyst upstream of a conventional three-component-control catalyst to improve NO conversion on the fuel-lean side of stoichiometry were demonstrated in laboratory tests.