Two catalyst formulations were tested on a diesel engine. A Fourier Transform Infrared (FTIR) analyzer was used to measure concentrations of nitric oxide (NO) and nitrogen dioxide (NO2). Other exhaust gas components were measured by conventional analyzers.The results indicate that nitrogen oxides (NOx) undergo complex chemical reactions in the diesel oxidation catalyst (DOC). The increase of NO2 occurring at a certain temperature range is accompanied by a decrease of the total NOx emission. NO2 is probably an important itermediate product in the lean NOx catalyst reaction chain.