The present work combines X-ray Photoelectron Spectroscopy (XPS), catalytic tests, and density functional theory (DFF) modeling for determining the optimized Co edge decoration according to the stability and reactivity of the CoMoS active phase. Using the phase diagram and morphology of the CoMoS nano-crystallites determined by DFT calculations [Krebs et al. Catal. Today, this issue], the values of the Co/Mo ratio and the number of mixed Co-Mo sites are evaluated. DFr calculations of the 2p core level shift of Co at the edges with respect to the Co9S8 phase are compatible with the experimental positive binding energy shift measured by XPS on CoMoS catalysts. Moreover, the decomposition of the XPS spectral envelope is used to quantify the proportion of CoMoS phase and to normalize the catalytic activity in toluene-hydrogenation per edge site as a function of the Co/Mo ratio. The optimal Co/Mo ratio for the catalytic activity is found to reflect the presence of mixed Co-Mo sites at Mo-edge when considering the measured average size of particles and the calculated morphology. These results are helpful for an improved optimization of the promoter content in the CoMoS phase. (C) 2007 Elsevier B.V. All rights reserved.