Xanthate requirements for the lead rougher flotation of Hellyer ore approximately halved as the pH increased from 10 to 12. At pH 12 or after an addition of 200 g/t NaCN, size by size analysis showed an increased recovery for galena and sphalerite, while pyrite flotation remained at entrainment values. This suggested that xanthate may have adsorbed to a greater extent on galena and sphalerite as a result of decreased adsorption onto pyrite. Thermodynamic calculations and UV analysis of a pyrite pulp have shown that, in the pH range from 10 to 12, a transition occurs from a region of ferric-hydroxide xanthate stability to one of ferric hydroxide or ferri-cyano complex stability, resulting in a decreased adsorption of xanthate onto pyrite.