Hydrodemetallisation (HDM) of Ni-TPP was studied in a fixed-bed reactor at temperatures ranging from 550-615 K in the presence of a sulphided Mo/Al2O3 catalyst, The kinetic network of Ni-TPP HDM comprises hydrogenation steps and a lumped hydrogenolysis step. It appears that these two types of reactions occur on sites with different characteristics. Initial catalyst deactivation takes place in two distinct stages. The first deactivation stage is the result of nickel deposition and its end is marked by a relatively constant molar ratio of deposited nickel to molybdenum. Hydrogenolysis is affected to a larger extent by nickel deposition than hydrogenation. In the second stage deactivation is much slower. Deactivation follows first order kinetics in both stages. Finally, a quasi steady-state activity is reached. Despite a continuing deposition of nickel, the catalyst remains remarkably active. It is concluded that nickel is not deposited on active sites. A variety of adsorbed species was found to exist on the working catalyst. Kinetic analysis of data sets obtained from experiments at steadystate conditions revealed that product inhibition of the hydrogenation reactions is negligible.