The present wording on implementing regulation on type-approval of motor vehicles and engines with respect to emissions from heavy-duty vehicles in Europe (e.g., Euro VI requirements) describes the procedure to determine gaseous emissions from on-road measurements using Portable Emissions Measurement Systems (PEMS).The gaseous emissions to be measured according to the PEMS protocol includes carbon monoxide (CO), total hydrocarbons (THC) and nitrogen oxides (NOx) for diesel engines and with the addition of methane (CH₄) for gas engines. Measurement methods of particles are at this stage under development. Emission data from on-road testing can be used for calculation of In-service Conformity (IsC) pass/fail criteria but also for establishing local emission factors for specific routes.The procedure for the measurement is described in detail in the regulation. During on-board measurement, emission sampling, measurement of the exhaust parameters and recording of the engine parameters as well as ambient data shall continue throughout the normal in-use operation of the vehicle, i.e., driving conditions should correspond to actual real-life conditions.Certification testing of HD engines is carried out in a very controlled laboratory environment by testing an engine as a standalone unit. Results from real-life emission testing might thus be very different from testing according to the certification procedure. This paper will present the impact on emissions from four heavy-duty vehicles with regard to factors such as ambient temperature, engine coolant temperature as well as vehicle load. It will also highlight the variation in emission results with regard to various driving conditions using different calculation methods.