Mossbauer emission spectroscopy (MES) was used to study the influence of high pressure on the sulfidation of CoMo/C catalysts. During temperature stepwise treatment at 4 MPa the catalysts' behavior closely follows the Co-Mo-S model. The obtained MES spectra are different from those previously observed for CoMo/C catalysts sulfided under atmospheric pressure conditions, where the highly dispersed Co9S8-type structures are formed. It is suggested that the high-pressure conditions induce the binding of Co and Mo intermediate species, favoring Co-Mo-S phase formation. The results indicate that the contradictions between some catalyst models are partly related to the formation of different Co species upon different activation procedures. Lower stability compared with the alumina-supported catalysts was observed for the Co sulfide species in the CoMo/C catalysts under high-pressure sulfiding conditions. This can have important implications for the high-pressure stability of Type 11 Co-Mo-S. which is believed to have the same properties as the carbon-supported Co-Mo-S structures. (C) 2004 Elsevier Inc. All rights reserved.