The application of pulsed cavity ring-down spectroscopy has been demonstrated for the in situ quantitative determination of NO and NO2 in the exhaust of a diesel engine. NO absorption has been monitored at the transition from the X²¿ ground state to the A²S+ state at 226 nm. For NO2, absorption bands in the spectral region from 438 nm to 450 nm were used. At the selected engine conditions, concentrations of 212±22 ppm and 29 ± 4 ppm have been measured for NO and NO2, respectively, in good agreement with separate chemical exhaust gas analysis. The method is sensitive enough to meet the European Euro V standard directive on NOx emissions. This communication discusses the relatively simple setup needed for this type of measurement, the problems encountered, as well as the prospects for single-stroke, simultaneous measurements of both NO and NO2 at the sub-ppm level.