From our operando XAS monitoring, we propose that the investigated L-CoS exhibits the following structural and reaction kinetics toward the acidic HER: (i) L-CoS maintains its structural integrity well for catalyzing the HER under acidic conditions, as supported by the unchanged local electronic structure from the operando XAS analysis (Figure ); (ii) investigation of operando XANES spectra unveil a kinetic change in the Co valence state, whereas the formation of low-valent Co species is dominant in the HER (Figure ); (iii) analysis of operando FT-EXAFS spectra demonstrate that the HER sets along with an electrochemically driven structural optimization of the S-Co-S-Co-S moieties. All these suggest that the in situ optimized low-valent S-Co-S-Co-S moiety is critical to trigger the HER of L-CoS under acidic contains. Further KSCN poisoning tests (Figure ) reveal that the acidic HER of L-CoS proceeds with the adsorption/desorption of H* intermediates at the terminating S sites in the S-Co-S-Co-S moieties. Additionally, combination of in situ Raman (Figure ) and operando XAS (Figure ) characterizations indicate that the mentioned electrochemically driven structural optimization does not alter the local Co coordinations. However, we believe that such an optimization shows strong correlations with the metal redox behaviours and the structural distortion of [CoS6] octahedral. These factors can also play a role in tuning the HER kinetics of L-CoS in 0.5 M H2SO4.