Layered transition metal (TM) sulfides like MoS2 (WS2, ReS2) are widely studied in reactions involving H2. The edges of MoS2 slabs are currently viewed as the active sites. However, along with slabs, small clusters and single atoms (SAs) are present, with unknown structure and activity. Our research is focused on the structure and activity of these subnanometric sulfidic TM entities. Such species might be adsorbed on the support or be embeded into a framework (e.g. of a MOF or of N-doped carbon)1. In this work we applied different syntheses to obtain ultradispersed Mo sulfide species, studied their activity in thiophene hydrodesilfurization (HDS) and hydrogen evolution reaction (HER), then followed their evolution during the reactions by XAS operando. The catalysts (1 wt.% Mo) were obtained via dry impregnation/adsorption or via one-pot routes. Various supports (activated carbon (AC), TiO2, SiO2, Al2O3) and various one-pot techniques were used. The systems were sulfided in H2S/H2 flow and characterized by TEM, UV-vis DRS, Raman, XRD, STEM-ADF and tested in the HDS of thiophene and in the electrochemical HER. The evolution of Mo species during sulfidation and during the HDS and HER reactions was investigated by operando QXAS at the ROCK beamline of SOLEIL synchrotron (for HDS) and operando HERFD XANES at the FAME-UHD beamline of ESRF synchrotron (for HER). The adsorbed few-atom clusters are exemplified by the MoSx/AC catalyst and incorporated single atoms - by a phthalocyanine polymer MoSx-Pc. While MoSx/AC demonstrated high specific activity per Mo atom in HDS and high current densities in HER, MoSx-Pc was not active. Physico-chemical techniques (Raman, STEM..) attest that few-atom clusters and SAs are present in MoSx/AC,2 whereas MoSx-Pc contains only SAs. Operando QXAS has shown that Mo species on AC during sulfidation are transformed into sulfidic oligomers with 2-3 Mo atoms. In the MoSx-Pc, Mo remains single-atomic, surrounded by 4 N and 2 S atoms . Both species are stable during the HDS reaction. When exposed to HER conditions in the acidic media the adsorbed MoSx are oxidized and in MoSx-Pc sulfur is replaced by oxygen. Our results suggest that both HER and HDS reactions require the presence of several (at least 2) Mo atoms in the active site, whereas fully single-atomic MoSnNm species are inactive. [1] Chen, Y. Et al. Joule 2018, 2, 1242-1264. [2] Ryaboshapka, D. et al. Appl. Catal. B :Environ. 2022, 302, 120831.