The hydrophobic character of a variety of base-metal sulfides and gold has been examined in the presence of sodium hydrosulfide and potassium ethylxanthate. The degree of hydrophobicity induced by electrodeposition of sulfur and by electrochemical reaction between the minerals and ethylxanthate were determined by contact-angle measurements on polished surfaces and by flotation of mineral particles. During flotation, mineral-reagent interactions were followed by monitoring the solution composition spectrophotometrically. Results have shown that on gold and chalcopyrite the oxidation of hydrosulfide to molecular sulfur and the oxidation of ethylxanthate to dixanthogen produced hydrophobic surfaces; on pyrite, sulfur deposition was more effective than dixanthogen in producing a hydrophobic surface. Chalcocite, in contrast to gold, chalcopyrite, and pyrite, was found to react directly with ethylxanthate and hydrosulfide, but only the metal xanthate product was hydrophobic.