Several factors must be taken into consideration while interpreting data on kinetics of reactions occurring during hydroprocessing of petroleum residues, i.e., the origin of feed, properties of catalysts and related diffusion phenomena, catalyst deactivation, form of kinetic model, and experimental system employed. Among the operating parameters, temperature, H2 pressure, H2S/H2 ratio, H2/feed ratio, and contact time have a pronounced effect on kinetic data as well. Moreover, although of the same distillation cut point, residues may vary widely in composition. For example, the content of metals (vanadium+ nickel) may range from few ppm for residues derived from a sweet crude to more than 1000 ppm for one derived from a heavy crude. The content of asphaltenes and CCR may exhibit a similar variability. For the most part, significant discrepancies among reported values of kinetic parameters can be reconciled by closely examining the conditions of experiment used for determination of kinetic data. Kinetic data are incorporat ed in mathematical models used for predicting the life of catalyst during hydroprocessing operation. Good predictions can be made if reliable kinetic data are used for modeling. In this regard, the selection of experimental conditions for determining kinetic results is crucial to ensure their reliability. Thus, properly designed tests for accelerated aging of catalyst may be the source of a valuable information. Otherwise, erroneous conclusions could be reached.