The synthesis of bimetallic Co-Mo compounds from ammonium heptamolybdate, citric acid and cobalt acetate for the preparation of catalysts for the ultra-deep hydrodesulfurization (HDS) of diesel fuel is reported. The structure of the Co-Mo compounds formed in solution and on alumina surfaces was studied by Mo-95, O-17, C-13, and Al-27 NMR, FTIR, Raman and XAS spectroscopy. It was found that the oxidic precursor of the catalyst consists of tetrameric molybdenum (VI) citrate anions with Co2+ cations coordinated to the carboxyl groups and terminal oxygen atoms. After sulfidation, the prepared catalyst was tested in the HDS of straight run gas oil, demonstrating high activity in the production of ultra-clean diesel fuel. (C) 2010 Elsevier B.V. All rights reserved.