Simulated distillation data of distillates produced by hydroconversionof a vacuum residuehave been described as a two-component cumulative normal distributionfunction. The influenceof catalyst addition upon calculated parameters of the model indicatesthat distillate fraction isproduced through a consecutive mechanism. A modifiedthree-component model has then beenapplied. It reveals that catalyst limits the extent of the firstbond cleavage step and reduces theyield of light distillate. Furthermore, the distribution ofhydrogen consumption is strongly affectedby catalyst addition, which favors hydrogen incorporation indistillate.