Mechanistic studies of the initial decay of hydrodemetallization (HDM) catalysts are performed in a fixed-bed reactor using metalloporphyrins as model compounds. Adsorption of the metalloporphyrins on alumina is quantitatively confirmed, and the metal deposition is proven to have some autocatalytic activity especially hydrogenation activity instead of being a poison. The autocatalytic activity is increased with the metal accumulation on alumina to some extent. However, these metal deposits are still far inferior to commercial HDM catalysts for removing the metals from the stream. Instead of interpreting the initially declining activity as a result of catalyst deactivation, a simultaneous adsorption and reaction mechanism is first proposed in this study. The initial apparent activity is an effect of a nearly constant catalytic activity of the active phase coupled with an extra decreasing adsorption on the alumina support. Consequently, the model based on the new mechanism fits the experimental data reasonably well. (C) 2003 Elsevier Inc. All rights reserved.