A computer model was developed to compare a conventional cocurrent trickle-bed reactor with two novel countercurrent three-phase reactors: the three-levels-of-porosity reactor and the internally finned monolith reactor. The hydrodesulfurization of a vacuum gas oil was taken as a case study. It was found that the application of countercurrent flow in the novel reactors results in significant increase of conversion. Sensitivity for mass transfer and H2S inhibition were investigated. Reduced mass transfer compared to a conventional trickle-bed does not limit the reaction.