The hydrodesulfurization (HDS) activity of phosphorus-added CoMoS/Al2O3 catalysts has been examined using dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) as reactants. Phosphorus modifies catalytic behavior in two ways: increase of active sites by enhanced metal dispersion and increase of Bronsted acidity. These effects yield a different distribution of products depending on the reactants. In thiophene HDS, phosphorus increases only the conversion. In DBT HDS, phosphorus increases the conversion and produces more cyclohexylbenzene (CHB) than biphenyl (BP). In 4,6-DMDBT HDS, the conversion also increases but more dimethylbiphenyl (DMBP) is produced than methylcyclohexyltoluene (MCHT), the opposite trend from the one observed in DBT HDS. This is due to enhanced acidity of the catalyst, which allows migration of the methyl groups in 4,6-DMDBT. Maximum conversions are obtained when the phosphorus content is proper, 0.5 wt.% in this study.