In this work we examine the effect of different catalyst particle shapes (sphere, pellet, cylinder, 2-lobe, 3-lobe, and 4-lobe) on the characteristics and behavior of hydrodesulfurization catalyst and reactor, such as liquid holdup, external and internal concentration gradients, sulfur content in the product, and pressure drop. The analysis was based on experimental information obtained in a small hydrodesulfurization reactor using straight-run gas oil as feed and a tri-lobular commercial catalyst sample at typical commercial operating conditions. An isothermal reactor model is described in detail, which was validated with experimental information and then applied to study the effect of the different particle shapes on HDS reaction. (C) 2004 Elsevier B.V. All rights reserved.