The role of dextrin in the xanthate flotation of pyrite and sphalerite was examined by means of flotation tests and microelectrophoretic measurements. Floatability and zeta-potential of both minerals were found to be depressed in a suitable pH range due to the formation of their respective superficial metal hydroxide layers. The influences of surface oxidation and flocculation of mineral particles were also discussed. The selective flotation of sphalerite from pyrite in an acidic medium in the presence of copper sulfate was found to be possible, as shown by artificial mixture separation studies. In this way, the possible exclusion of cyanides (for environmental reasons) may be advanced.