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The pathbreaking papers by Held and Iwamoto has led to a reinvigorated search for lean NOx catalysts worldwide. Extensive effort is currently being expended to develop precious metal lean NOx catalysts, with application intended for diesel engines and low-temperature lean-burn gasoline engines. Here we concentrate on the properties of such a catalyst, referred to as LNX3. The NOx reduction selectivity of LNX3 is examined with model reducing agents. Hydrocarbons are more effective at reducing NOx than CO or H2. Next it is shown that excessively high HC/NOx ratios can degrade the performance of noble metal lean NOx catalysts. This is because the exotherm from the hydrocarbon oxidation can increase the temperature in a significant portion of the catalyst bed beyond the effective temperature window wherein the catalyst is able to reduce NOx.