The influence of high pressure on the sulfidation of calcined CoMo/Al2O3 catalysts, under conditions relevant to industry, was studied by means of Mossbauer emission spectroscopy (MES). The measurements were carried out in a high-pressure Mossbauer in situ reactor, which offers the possibility of studying cobalt-based catalysts under realistic hydrotreatment conditions. Activation at 673 K and 4 MPa results in complete sulfidation of Co that has diffused into the Al2O3 support during calcination, the Co-Mo-S structure being the only Co phase present in sulfided industrial CoMo/Al2O3 catalysts. On the basis of comparisons with previous MES results on alumina-supported catalysts it is concluded that the calcination treatment retards the sulfidation of the Co atoms to temperatures where MoS2 particles are already present, favoring Co-Mo-S phase formation. (C) 2003 Elsevier Inc. All rights reserved.