In this study, L-CoS emerges as an effective catalyst for the HER/OER under different reaction conditions. Recent in situ/operando studies placed emphasis on tracking the structural transformation of sulfide into (oxy)hydroxide, while debates are still going on as to whether the formation of high-valent metal centers and the presence of oxidized residual anionic species are both the key to the OER. Most importantly, a precise elucidation of the nature of the true catalytically active sites and species of sulfide-related catalysts under HER conditions remains to be performed. Herein, we implemented operando XAS techniques to identify the structural and reaction kinetics of cobalt sulfidebased catalysts for both the HER and OER under different reaction media. First, for the HER under acidic conditions, analysis of operando XAS characterization shows that L-CoS retains its initial structural integrity throughout the entire cathodic polarization process. Comprehensive interpretations of operando FT-EXAFS spectra reveal that the local geometry of Co centers undergoes electrochemically driven optimization into low-valent Co-S-Co moieties, which favor the adsorption/desorption of key HER intermediates at the terminating S sites in Co-S-Co moieties.