Hydrodesulfurization activity of cobalt Chevrel phase sulfide catalysts has been investigated in a fixed-bed flow reactor. Single phase cobalt Chevrel phase sulfides exhibited catalytic activity for hydrodesulfurization. Supported cobalt Chevrel phase sulfide catalysts indicated much higher hydrodesulfurization activity and even higher than commercial CoMoS/Al2O3 catalysts. The existence Of Mo6S8 and the reduced oxidation state of Mo in the cobalt Chevrel phase sulfides has been observed by XRD, LRS, and XPS.