This work is dealing with model studies on catalytic hydrodemetallization of residual oils. Pure nickel porphyrins dissolved in decalin were converted over sulfided or reduced hydrotreating catalysts. For this purpose, two flow type high pressure units with fixed bed reactors were constructed. Liquid product samples could be sluiced out in short intervals. Atomic absorption and UVNIS spectroscopy as well as capillary gas chromatography were routinely employed for product analysis. The time on stream behavior of the model systems and catalyst deactivation were thoroughly investigated. Partially hydrogenated, metal containing intermediates and various metal-free degradation products were detected. This enabled new insight into the mechanism of hydrodemetallization.