The search for catalysts capable of reducing NOx in the exhaust of internal combustion engines operated in the lean-burn mode is presently focused on copper-exchanged zeolites. Despite intensive study, the mechanism of reduction over these catalysts is not well understood. In this study new complications are revealed. Specifically, it is demonstrated in a typical synthetic exhaust stream (containing hydrocarbons and oxygen) NO is rapidly, and in some cases nearly completely, homogeneously (no catalyst present) converted to NO2. Under some conditions N2O is also formed. It is also shown that most of the hydrocarbons are homogeneously oxidized in these same streams in the temperature range in which lean-burn catalysts are expected to operate. These processes only take place in gas mixtures containing both hydrocarbons and oxygen. Moreover, the extent of NO conversion to NO2 is shown to be a function of temperature, the hydrocarbon to NO ratio of the input stream, the input CO concentration, and flow rate. In short, homogeneous chemistry is in some cases a significant factor in catalytic NOx reduction.