The paper discusses the issues related to the measurement of the operating parameters of a conventional engine and the parameters of a conventional bus in motion. The measurements were performed under actual traffic conditions of a city bus in Poznań. It is noteworthy that onroad tests under actual operating conditions (PEMS) are one of the main trends of the development of the testing methodology and are gradually gaining in importance. Specific conditions were selected to ensure the highest possible reflection of the actual conditions: the traffic similar to the traffic typical of a regular bus route (average traffic on Poznań routes). The tests were conducted for two days (Friday and Saturday), characteristic of very high and very low number of transported people respectively as well as high and medium traffic congestion. During the tests the engine and the driving parameters were recorded continuously as sent from the diagnostic system of the vehicle and the GPS. The PEMS test method (portable emissions measurement system) presented in the article can be used in the measurement of exhaust emissions in rail vehicles with combustion engines.