A procedure to determine the exhaust gas emission factors for the fleet of heavy duty vehicles in the Federal Republic of Germany is reported on. The exhaust gas emission factors for carbon monoxide, unburned hydrocarbons, nitrogen oxides and fuel consumption are indicated separately for light commercial vehicles (maximum permissible weight not exceeding 3.5 t) and heavy duty vehicles (maximum permisible weight exceeding 3.5 t). With the particle emission factors, a distinction was made beteen the total particle mass (carbon + associated organic substances) and particle mass free of hydrocarbons. As for the heavy duty vehicles, appreciable differences were observed in the partial emission factors of the various mass classes. The application of the procedure thus requires that the frequency distribution of the mass classes is known.