Sulfiding medium strongly affects the performances of unsupported metal sulfide hydrodesulfurization catalysts. A gas-sulfided CoMo unsupported catalyst has about the same activities for desulfurizing dibenzothiophene and 4,6-diethyldibenzothiophene. Its liquid-sulfided counterpart desulfurizes 4,6-diethyldibenzothiophene much faster than it desulfurizes dibenzothiophene. By contrast, 4,6-diethyldibenzothiophene desulfurizes far more slowly than dibenzothiophene on Al2O3-supported sulfide catalysts. The implications of these results are discussed.