Dispersion and thermal behaviors of oil-soluble Mo-dithiocarbamate (Mo-DTC) and Mo-dithiophosphate (Mo-DTP) as the MoS2 catalyst precursors were studied in petroleum vacuum residue (VR) using FT-Far IR, XRD and TEM. FT-Far IR was proved to detect the Mo complexes and their derived MoS2 in VR without the interference of the complicated organic matrix. Their transfon-nation into MoS2 was identified by detecting the changes in the ligand bonds and crystal structure. These complexes were found to be distributed in asphaltene (or maltene) by fractionation with hexane due to their solubility in the heavy oil, ruling out any chemical interaction of the complex with asphaltene. Mo-DTC was found to be decomposed at 350degreesC to form definite MoS2 in VR. Mo-DTP started its decomposition around 200degreesC, and however, no definite formation of MoS2 was confirmed by heating up to 500degreesC. Dispersion of the complexes in VR and asphalthene was always good as indicated by TEM. Both complexes in VR at 380Cdegrees under hydrogenation (HYD) conditions provided more or less MoS2. H2S and reactive sulfur species were assumed to accelerate the transformation of the Mo complex to MoS2 during HYD reaction. Some difficulty of Mo-DTP to be transformed quantitatively into definite MoS2 in VR may explain its poor activity for up-grading VR. (C) 2002 Published by Elsevier Science Ltd.