Concerning the design of new catalysts, our results unveil distinct differences in the restructuring processes between sulfide and oxide-based catalysts, which suggests that O incorporation strategies can be promising for further optimizing the sulfide catalysts for overall water splitting. Our operando results highlight the crucial role of the kinetic transformation of low-valent S-Co-Co-S in the alkaline HER of L-CoS. This furthermore inspires future studies on the alkaline HER of sulfide-related catalysts via secondary metal incorporation to optimize these reactive S-M-M-S (M: transition metals) configurations. We expect our comprehensive understanding of the catalysts under diverse reaction conditions to inspire new (re-)investigations of true structure-activity relationships, which are essential to design effective water-splitting catalysts in the near future.