In this paper we describe a new model of catalyst deactivation due to riser reactions in a FCCU. For this purpose, we analyse different coke types and model the formation of each type based on oil feed properties rather than considering all cokes as being equal like other authors do. Thereafter, we examine different deactivation kinetic equations and propose an alternative that describes catalyst deactivation against coke as opposed to against time as in conventional approaches. Although it is easier to model deactivation against time, our model is more flexible and intuitive since it can simulate special situations, such when the catalyst is partially regenerated. (c) 2006 Elsevier B.V. All rights reserved.