Hydrodesulfurization (HDS) of dibenzothiophene (DBT) catalyzed by alumina-supported ruthenium complexes was investigated in a pressurized flow system. The catalysts prepared by activation of ruthenium carbonyl (Ru3(CO)12)-triethylamine(NEt3)-ethanethiol(EtSH)/Al2O3 systems where metal carbonyls were treated with NEt3 and EtSH to give anionic complexes with metal-sulfur bonds were found to be active for hydrodesulfurization (HDS) of dibenzothiophene (DBT). When the Ru3(CO)12-NEt3-EtSH/Al2O3 system was activated in a H2S, H2, or N2 stream, the catalytic activity was higher than that of the catalysts derived from alumina-supported RuCl3, Ru(acac)3 (acac=acetylacetonate), Ru(COD)(COT) (COD=cyclooctadiene; COT=cyclooctatriene) and Ru3(CO)12 and molybdena-alumina. When Ru(0) complexes such as Ru(COD)(COT), Ru3(CO)12 and anionic ruthenium carbonyls were used, the selectivity for biphenyl (BP) was more than 86% which was also higher than that observed with the use of a conventional molybdena-alumina catalyst (71%). From the results of NO and CO chemisorption, it was suggested that activation of a Ru3(CO)12-NEt3-EtSH/Al2O3 system would give highly dispersed ruthenium species. XPS measurement of catalysts before HDS revealed that Ru3(CO)12/Al2O3 was sulfided to a small extent (S/Ru ratio 0.83) while alumina-supported RuCl3 and Ru(acac)3 were sulfided up to S/Ru ratios 2.31 and 1.51, respectively. After HDS, however, S/Ru ratios of all ruthenium catalysts decreased to ca. 0.5, indicating that, under H2 pressurized conditions, the ruthenium species on alumina were nearly metallic rather than deeply sulfided. When the Ru3(CO)12-NEt3-EtSH/Al2O3 system was activated in a H2 stream, both S/Ru ratios before and after HDS were ca. 0.5, indicating that, in the Ru3(CO)12-NEt3-EtSH/Al2O3 system, the active species for HDS can be obtained during the stage of catalyst preparation.