Unsupported Co-Mo catalysts were prepared by impregnation Of MoS2, obtained by thermal decomposition of ammonium tetrathiomolybdate, with a solution of cobalt nitrate in acetone. XRD and HREM showed that the MoS2 structure of the resulting catalysts remained intact upon addition of Co. The thiophene hydrodesulfurization and the consecutive butene hydrogenation properties indicated that the Co ions are preferentially situated at the edges of the MoS2 Crystallites below Co/(Co + Mo) = 0.08 and promote the hydrodesulfurization (HDS) reaction. Further addition of Co leads to the segregation of Co9S8, which acts as a support for highly dispersed Co-promoted MoS2 Crystallites. As a result of this structural effect, the Co-MoS2 crystallites become better accessible and the catalytic activity increases again above Co/(Co + Mo) = 0.2. The observed increase in activity upon physical mixing of Co-promoted MoS2 and Co9S8 might also be explained by such a structural effect, rather.than by a remote-control effect.