A vacuum gasoil was cracked in the MicroDowner at various operating conditions. Cracking yields were simulated with a one-dimensional hydrodynamic model and pseudo-homogeneous kinetics. A new model for describing catalyst deactivation in gasoil cracking has been used, and compared with a more traditional model based on coke-deactivation. The new model includes a fraction named strippable coke that refers to molecules that are strongly adsorbed on catalyst surface (active site coverage), but which can be stripped out of the catalyst and recovered into the heavy cycle oil fraction after the reaction. At short time-on-stream this fraction represents the major cause of deactivation, and is slowly replaced by coke-based deactivation as coke yield increases on catalyst surface. (c) 2005 Elsevier B.V. All rights reserved.