Abstract: Catalyst life in desulfurization of distillate hydrocarbon streams is predicted with the aid of a computer simulation which embodies an analytical kinetic model of a hydrodesulfurization reaction and a semi-empirical model for catalyst deactivation. The simulation specifies the decline of catalyst activity with time and a current reaction temperature required to maintain the initial catalyst activity. The useful life of the catalyst is considered to be the time required for catalyst activity in the simulated reaction to decline sufficiently from its initial level so that a predefined maximum reaction temperature is reached while maintaining a desired level of sulfur in the desulfurized product. The computer time needed to simulate the reaction is decreased by combining superior features of LOTUS 1-2-3 and FORTRAN language, such that input/output and graphic operations are implemented with LOTUS 1-2-3 and numerical calculations are executed in FORTRAN.