This work is dealing with model studies on catalytic hydrodemetallization and hydrodesulfurization of heavy oils and residua. The catalytic hydrodemetallization was investigated in the presence of a sulfur component which is typical for petroleum. Model oils consisting of decalin, a nicke! porphyrin and dibenzothiophene were employed. The investigations were carried out in a flow-type high pressure unit with a fixed bed reactor on a sulfided Co0-MoO₃/Al₂O₃ hydrotreating catalyst. For analysis of the liquid product samples, which were sluiced out periodically in short intervals, atomic absorption spectroscopy, UVIVIS-spectroscopy, capillary gas chromatography and high pressure liquid chromatography were used. The degree of conversion for both reactions and the product distributions as a function of time on stream were thoroughly investigated. This enabled new insights into the mutual influences of hydrodemetallization "and hydrodesulfurization.