In-situ Mössbauer Emission Spectroscopy (MES) has been used to study the type of phases present in sulfided activated carbon-supported Co and Co-Mo hydrodesulfurization (HDS) catalysts. Most of the reported MES studies are performed on Co-Mo/Al2O3 catalysts. Co species in the so-called "Co-Mo-S" phase, sofar only observed in sulfided catalysts containing Co and Mo, should govern the HDS activity. The present observations show that the same Co species in sulfided Co/C and Co-Mo/C catalysts, with the same quadrupole splitting as "Co-Mo-S" can be formed. Furthermore, it turns out that the QS-value of the "active phase" in the sulfided Co/C and Co-Mo/C catalysts depends on the sulfiding temperature and Co content. Hence, it seems unlikely that there will be only one well defined active sulfide phase which governs the HDS activity.