Dextrins from thermal modification of potato starch under ammonia, hydrogen sulfide, and gluten were tested as flotation depressants of galena and chalcocite in a Hallimond cell with xanthate to make the sulfides hydrophobic. All dextrins, except prepared under hydrogen sulfide, depressed flotation of the sulfides. The cessation of flotation occurred at different doses of dextrins, usually between 2 and 20 kg per ton of the floated material. Dextrins from starches thermally modified under ammonia were found to be promising selective depressants in the studies with model (1:1) mixtures of galena and chalcocite but poor separation was achieved with copper ore containing lead minerals. The separation of galena from chalcocite form synthetic mixtures was also poor in the presence of gluten-modified dextrins.