To unravel the role of pH variations in the intrinsic HER kinetics, operando monitoring of three CoSe2 catalysts (i.e. o-CoSe2, c-CoSe2, and c-S-CoSe2) was also performed at pH 14. From our operando analysis, we observed a multi-step restructuring into metallic cobalt species in o-CoSe2 toward the alkaline HER. Specifically, during the pre-activation process, the interaction reaction between o-CoSe2 and the alkaline medium results in the formation of rich oxidized species covered on its surface; upon cathodic polarizations, these oxidized species further convert into metallic Co species, which serve as the true active species for the alkaline HER. DFT simulations corroborate that these in situ-generated metallic Co species in alkaline media exhibit a high energy barrier for the H2 release compared to acidic conditions. Consequently, o-CoSe2 displays poor HER performance at pH 14 than at pH 0. Notably, combined above operando spectroscopic monitoring and DFT calculations demonstrate that c-CoSe2 and c-S-CoSe2 are thermodynamically more stable than o-CoSe2. These catalysts retain their structural integrity well throughout the entire alkaline HER. On this basis, the as-prepared c-S-CoSe2 catalysts excel through excellent alkaline HER performance with a low overpotential of -121 mV (-10 m/cm 2 ) and robust electrochemical durability for over 300 h with any obvious changes.