Ultra-deep hydrodesulfurization (HDS) was attempted to achieve the sulfur level less than 15 ppm S by staged and layered catalyst beds over commercially available catalysts. It was confirmed that two-stage and two-layer reaction schemes can attain the targeted sulfur level. Two-stage reaction with hydrogen refreshment between stages could provide the sulfur content lower than 15 ppm S over most of the catalysts examined in the present study where the first stage desulfurized the feed oil to be of 300-400 ppm S. Two-layer reaction, where two layers were connected directly without gas renewal, suffered severe inhibition of H2S and NH3. Nevertheless, NiMo sulfides on alumina support with sufficient acidity or zeolite achieved the sulfur level lower than 15 ppm S by two-layer scheme. Nitrogen species survived in the first layer reaction were found to strongly affect the extent of desulfurization in the subsequent second layer reaction. (C) 2003 Elsevier B.V. All rights reserved.