Due to a low content of H-2 in the makeup gas which was added into the recirculation stream, the hydrodesulfurization ( HDS) efficiency of the commercial unit is continuously decreasing. To maintain the required level of sulfur in the products, both elevated temperature and a space velocity decrease were applied. This situation ends with a severe capacity reduction. The observed decline in HDS efficiency is due to the disturbed H-2/H2S ratio in the recycle gas. This was confirmed by the pilot- plant studies of the influence of the reaction parameters on the HDS efficiency as a function of H-2 purity in the gas phase.