The goal of the present work was to investigate the ability of the SEMTECH®-D Portable Emissions Measurement System (PEMS) to provide simultaneous, accurate, real-time (1Hz) measurements of NO and NO2 in vehicle exhaust. Extensive chassis dynamometer laboratory evaluation studies of the SEMTECH® system were conducted. The instantaneous (1Hz) NOx emissions were measured using a conventional chemiluminescence analyzer (CLA) and were compared to the sum of the instantaneous NO and NO2 measurements from the SEMTECH®-D. The sum of the NO and NO2 emissions measured by the SEMTECH® were in excellent agreement (within 95% in most cases) with the total NOx measurements from the conventional CLA.During the laboratory evaluation studies, several Federal Test Procedure (FTP) drive cycles were conducted. Examples of the NO and NO2 concentration and mass emissions measured using the SEMTECH®-D are presented along with the corresponding SEMTECH®-D detection limits. From the data it is clearly evident that the NO and the NO2 levels measured were, in most cases, well above the detection limits of the SEMTECH®-D. The results also indicate that for the vehicles tested in this study, the SEMTECH® system can easily distinguish differences between NOx, NO, and NO2 emissions measured during cold start transient phase (Bag 1) and the hot start transient phase (Bag 3) of the FTP.The SEMTECH®-D PEMS utilizes a sample gas conditioning unit prior to the analyses of NO and NO2. Therefore, studies were also conducted to evaluate potential losses of NO2 via adsorption, condensation, and reactions with the water that may accumulate in the condenser of the sample conditioning unit. The results of these studies indicate that for NO2 concentrations of less than ∼300 ppm (the range of NO2 typically emitted from the vehicles tested) the losses in the SEMTECH® system were comparable to those losses of NO2 that may be attributable to NOx converter efficiencies (> 95% efficient, as specified in the Code of Federal Regulations) in conventional laboratory analyzers (1).