The Co-Mo/gamma-Al2O3 catalysts promoted with P and Mg were prepared and found to be highly active and stable for the hydrodesulfurization of commercial naphtha. The catalytic behavior of hydrogenation and isomerization of such catalysts were studied using model naphtha. The catalysts were found to be highly active for the hydrodesulfurization (HDS) of thiophene in hexane, but the activity was significantly inhibited by the presence of olefin. The model compound 1-hexene was found to be isomerized (mainly a double-bond shift with a little skeletal isomerization) and hydrogenated directly to n-hexane at the comparable rates over the sulfided Co-Mo/gamma-Al2O3 catalysts. Microcalorimetric adsorption of ammonia showed the fairly strong surface acidity for the Co-Mo/gamma-Al2O3 catalysts, which might be responsible for the isomerization activities. In addition, the hydrogenation and isomerization of 1-hexene seemed to proceed in a competitive way and were affected strongly by the solvents. The activity of hydrogenation was higher, while that of isomerization was lower for 1-hexene in heptane than in benzene. This result could be explained if it was supposed that the hydrogen transfer was the main pathway for the hydrogenation of olefins, considering that heptane is a better hydrogen donor than benzene. A pathway was also demonstrated in this work that I-hexene could be first isomerized and then hydrogenated to isoalkanes of higher octane numbers. The other double-bond isomers generated from 1-hexene over the acidic sites were more difficult to be hydrogenated to n-alkanes, which might also reduce the octane number loss of naphtha during the hydrodesulfurization.