The legislative pressure is being increased to reduce maximum allowable sulfur content in diesel to 50 wppm and lower. Existing hydrodesulfurizers currently reducing sulfur level to 500 wppm will not be able to meet this future requirement. Some improvements can be made in these existing units by switching over to new generation of catalysts and changing the levels of various process variables. Installation of a second reactor to accommodate additional catalyst volume will probably be inevitable for meeting this requirement of new sulfur specification. In this article, the various issues related to the second reactor, such as selection of a proper catalyst, optimum level of process conditions, location and operating mode of the reactor are discussed. Various process configurations based on the location of the second reactor have been analyzed. Different options for operating the second reactor under a lean atmosphere of H2S have also been discussed. All these process configurations differ in terms of capital investment, operating cost and desulfurization efficiencies. Future studies are required to take the optimum decision. (C) 2003 Elsevier B.V. All rights reserved.