Two mechanisms, a classical poisoning mechanism of the metal deposits and a simultaneous adsorption and reaction mechanism newly proposed in our preceding study, are applied to model the performance of two hydrodemetallization (HDM) catalysts in the aging experiments with Venezuelan vacuum residue. By interpreting the initially decaying activity as an effect of decreasing adsorption on alumina supports rather than deactivation of original active sites, the new model follows the trends of the observations much better. Moreover, the typical characteristics of the poisoning model with autocatalytic effects are also part of the new model: the initial stage of decaying is related to the metal concentrations of the feed and the monolayer coverage capacity of the catalyst, and after this period there is a residual activity left. Therefore, the simultaneous adsorption and reaction mechanism is good and convincing enough to explain the initial decay of the HDM catalysts with real petroleum feeds. (C) 2003 Elsevier Inc. All rights reserved.