Research has been carried out on the formation of cobalt (Co) and molybdenum (Mo) complexes in CoNlo-solutions, which are used for the preparation of Al2O3-supported hydrodesulphurization (HDS) catalysts. Aim of this study was to obtain more insight into possible Co-Mo interactions and their implications for the impregnation process. For this purpose, Raman and UV-vis-NIR spectroscopy has been carried out on different impregnation solutions at various pH. In addition, multivariate curve resolution (MCR) of the Raman spectra has been applied to obtain information on the molecular structure and speciation of the different complexes present in solution. Furthermore, Raman microscopy and SEM-EDX were applied on Al2O3 extrudates after impregnation, to obtain information on the distribution of the different metal-ions into the catalyst body as a function of time. It is demonstrated that an interaction between Co(II)-cations and different molybdate anions is indeed present in CoMo-solutions. Two different CoMo-complexes are proposed in which Co2+-cations are bound to the outside of HxMo7O24(6-x)- complexes, resulting in a simultaneous transport of the Co and Mo metal-precursor complexes. Since Co acts as a promotor, a close interaction between Co and Mo in the final catalyst is of major importance for the catalytic activity. As such, the interaction should also be taken into account in the preparation of supported CoMo-HDS catalysts. (C) 2007 Elsevier B.V. All rights reserved.