As reported, the HER in different media can proceed via different reaction pathways, namely the Volmer step for the adsorption of H atoms and the Tafel/Heyrovsky step for yielding H2 gas. Nevertheless, the catalysts in the alkaline media generally exhibit poor HER activity compared to acidic media, which is mainly due to the sluggish water dissociation kinetics involved in the alkaline media. To address these intrinsic activity gaps of catalysts in different media, phase engineering has been demonstrated as an effective strategy. Inspired by this knowledge, we have conducted the HER measurements with other as-prepared highly crystalline cobalt sulfides, namely CoS2, CoS/CoS2, CoS, and CoS/Co9S8 (Figures ). The results reveal that under acidic conditions the HER activities of the sulfide catalysts follow the order CoS2 > L-CoS > CoS/CoS2 > CoS > Co9S8/CoS, while their HER activities show negligible changes under alkaline conditions. This suggests that regulation of crystal phases only holds the key to fast HER kinetics of sulfide catalysts in acidic media, but not in alkaline media. To explore the intrinsic HER kinetics of sulfide catalysts, we further performed the CV tests in different media. Based on our evidence (Figures ), all the sulfide catalysts underwent irreversible changes in their CV curves under alkaline conditions, while reproducible CV curves were obtained under acidic conditions. These results altogether point out that sulfide catalysts potentially involve disparate kinetics of their structural transformations in different media, which calls for a precise unveiling of the nature of their true catalytically active species.