A mathematical model has been developed for predicting the effects of various parameters such as recycle rate, permanent and temporary deactivation rate constants on catalyst activity. The intent of this analysis is to evaluate the importance of the ease with which a catalyst can be regenerated and to identify important catalyst properties associated with regeneration. A reactor (hydrotreater)-regenerator system having equal catalyst inventory in each unit is used as a basis for modeling. A fixed amount of fresh catalyst supply and spent catalyst withdrawal are assumed to be applicable to the reactor. The analysis shows that a small percent increase in the permanent deactivation rate constant relative to total deactivation rate constant decreases the mean activity of the catalyst in the reactor significantly. In most cases, the mean activity of the catalyst increased with increasing recycle rate. The effect of regeneration rate constant on the catalyst activity was not significant compared to that of permanent deactivation rate constant. 13 references, 3 figures.