Molybdenum disulfide is a widely used catalyst for hydrodesulfurization (HDS) of petroleum fractions. The current view attributes HDS catalytic activity to the edges of nanoscopic MoS2 slabs. However, some experimental results indicate that the description of these systems should be improved, namely with the help of recently emerged concept of ultradispersed metal-based catalysts. Along with MoS2 slabs, small subnanometric species are present in the systems which can act as catalytic sites for HDS [D. Ryaboshapka et al., Appl. Catal. B 302 (2022) 120831]. In this work we studied HDS catalytic performance and active species of ultradispersed catalysts with 1 wt.% Mo, prepared on different supports: TiO2, SiO2 (KIT-6), Al2O3 and activated carbon. It might be expected that Mo species on oxides would bond strongly with surface oxygen and remain single-atomic, whereas Mo species would bond weakly with carbon surfaces and form larger clusters. Conventional characterization techniques (TEM, Raman, XRD, UV-vis, DRS) confirmed that no MoS2 nanoparticles were present in the samples. On the HAADF-STEM images, small Mo clusters along with single atoms were observed. The species formed during sulfidation and HDS reaction were monitored by means of operando XAS at the ROCK beamline of SOLEIL synchrotron. The differences in sulfidation trends between the samples were compared. All the catalysts demonstrated high specific activity in HDS of thiophene (Fig. 1b). The differences in HDS rates and in selectivity were confronted to the evolution of Mo species depending on the used support. For the first time, it was shown that sulfided ultradispersed Mo species supported both on oxides and carbon are active in HDS. Depending on the nature of the support, the strength of metal-support interaction changed, impacting the nature of species formed on the support surface.