The flotation response of cerussite and galena treated with sodium sulfide solutions open to the atmosphere was assessed at pH 9.5 using amyl xanthate as the collector Since both the hydrosulfide and xanthate anions are electrochemically active, the redox potential of the suspensions was measured with a smooth-platinum/saturated calomel electrode system. Mineral flotation depends on the sulfide dosage and is strongly affected by the potential. Cerussite flotation is poor under sulfide-deficient and sulfide-rich conditions. Low sulfide dosages are consumed by aqueous lead dissolved from the mineral and precipitate as lead sulfide. These colloidal particles seem to deposit onto the cerussite surface and react with the collector. As a result, a peak in the flotation curve is noted. High sulfide dosages oversulfidize the mineral surface, shift the potential to reducing conditions, and depress the mineral. At two different collector additions, optimum flotation of sulfide-treated cerussite occurs in the potential range from -100 mV to about 150 mV. Galena flotation ceases above a certain sulfide dosage that lowers the potential to reducing values thus inhibiting the chemisorption of xanthate onto the mineral surface. The results of this research work indicate that the xanthate flotation of sulfidized, mixed oxide-sulfide lead ores may be controlled by monitoring the pulp potential.