The real thermal stability of the Co-Mo-S active structure in Co-Mo hydrodesulfurization catalysts was investigated for the first time by removing the effect of sintering of MoS2 particles by means of a CVD technique using Co(CO)(3)NO as a precursor of Co. The Co-Mo-S structure is thermally stable to the treatment at 623 K, slightly unstable at 673 K, and remarkably destroyed above 773 K, irrespective of the support. It is suggested that at a high temperature, part of the Co atoms is detached from the Co-Mo-S structure, leaving some MoS2 edge sites vacant, while the detached Co atoms form catalytically inactive cobalt sulfide clusters. The catalyst support affects the thermal stability of the Co-Mo-S structure; the Co-Mo-S structure supported on Al2O3 is thermally more stable than that supported on either boron-modified Al2O3 or SiO2. Quantitative calculations suggest that at 873 K about 30% of the Co-Mo-S structure supported on Al2O3 is decomposed after a 2 h treatment in a 10% H2S/H-2 stream, in contrast to about 50% of that supported on boron-modified Al2O3 and SiO2. Prolonged sulfidation of the Co-Mo catalyst supported on Al2O3 showed that at 673 and 773 K the Co-Mo-S structure was remarkably destroyed within the first 2 h, followed by a much slower decomposition during further treatment.