To elucidate the mechanisms of sulfide mineral depression, the U.S. Bureau of Mines examined the depressant effect of hydrosulfide on flotation of copper sulfides, using electrochemical techniques and flotation tests. Hydrosulfide addition is commonly used to achieve separation of copper sulfides from molybdenite. Depression of chalcocite and chalcopyrite floated by ethylxanthate and o-isobutyl- n-ethoxycarbonylthionocarbamate collectors occurred in similar fashion by two mechanisms. Depression occurred if the addition of hydrosulfide was sufficient to shift the potential cathodically, and in the case of xanthate, partial desorption of collector was detected upon hydrosulfide addition. The second mechanism involved interaction of hydrosulfide with the collector-adsorbed copper sulfide surface with no shift in potential. Flotation of chalcocite and chalcopyrite beds held at constant potential above the collector desorption potential was depressed with hydrosulfide addition, supporting prevously published data which showed that the affinity of hydrosulfide for copper surface sites is much greater than the collector species. The level of mineral surface oxidation, surface pretreatment, and the presence or absence of air determine the amount of hydrosulfide required for depression.