Abstract. RUHR OEL GmbH is operating a Hydrotreater for combined treatment of both light and heavy gasoils at its Gelsenkirchen Horst refinery (Germany). Most of the hydrotreater feedstock is cracked material. While the light gasoils are desulphurized in order to send it to the domestic heating oil pool, the heavy gasoils are pre-treated for catalytic cracking. With respect to new fuel specifications as result of the European Auto Oil Program, the requirements of mogas and diesel has to be taken into account. Referring to the overall refinery structure the complexity of the feedstock situation is shown as well as the various modes of operation due to the refinery target setting. Interactions of feedstock and product qualities as well as their influence on operability and planning are indicated. Extended pilot plant studies has been used to identify the major constraints of operation and to develop vectors in order to implement into the LP planning model. Single feedstock components as well as combined feedstock has been tested. Analysis follow the most important parameters. Yields, conversion rates and hydrogen consumption has been considered.