The temperature-time sequences to maintain constant the concentration of one product In a two-parallel (competitive) exothermic reaction system undergoing catalyst deactivation are analyzed. Kinetics is described by a power law model and linear particle deactivation. A computational algorithm and a parametric study for adiabatic packed bed reactors are presented. It is found that temperature-time trajectories are strongly influenced by mean activity, and the selectivity. Experimental Information for Hydrotreating of vacuum gas oil was obtained keeping constant desulfurization, while hydrocracking and denitrogenation are function of time. Trajectories and final operating temperature are predicted with error below 2% compared to real operation.