Ex situ and in situ decomposition of the sulfur-containing precursors ammonium thiomolybdate (ATM) and nickel diethyldithiocarbamate (NiDETC) was applied for the preparation of SBA-15-supported NiMo catalysts. The catalysts were characterized with X-ray diffraction (XRD), N-2-physisorption and high-resolution transmission electron microscopy (HRTEM). The in situ activation performed in the presence of hydrocarbon solvent during the HDS of dibenzothiophene is more beneficial for the preparation of NiMo catalysts with a high HDS performance compared to the ex situ activation mode using a N-2/H-2 (10% H-2) gas flow at 773 K. Low MoS2 stacking, small MoS2 slabs and a less pronounced pore blocking present in the in situ activated NiMo/SBA-15 materials might be mainly responsible for the high HDS performance. In addition, the in situ decomposition mode is a "softer" reducing atmosphere and represents a beneficial condition for the generation of the catalytic active "NiMoS" phase. (C) 2010 Elsevier B.V. All rights reserved.