The interaction mechanism of sodium-diisobutyl dithiophosphinate (DTPINa) with galena and pyrite was investigated using zeta potential, microflotation and FTIR measurements. To evaluate metal ion effects on both minerals, sulfides were conditioned in both water solutions and solutions containing Pb2+, Fe2+ or Fe3+. Results show that sodium-diisobutyl dithiophosphinate adsorbs onto galena and pyrite by a chemisorption mechanism. Collector shows a pronounced affinity toward lead species, either in the mineral lattice or adsorbed on the mineral surface as metal-hydrolyzed species. Pb-ions adsorb onto pyrite, thus promoting collector adsorption and a decrease in selectivity. DTPINa shows less affinity toward iron species. (C) 2003 Elsevier Science B.V. All rights reserved.