The previous flotation experiments with heazlewoodite (\left(\mathrm{Ni}{3} \mathrm{~S}{2}\right)) and chalcocite (\left(\mathrm{Cu}{2} \mathrm{~S}\right)), two major constituents of the Inco matte, revealed that the heazlewoodite flotation was strongly depressed by dextrin in an alkaline environment, while chalcocite flotation was not. Adsorption tests show that while dextrin exhibits high affinity toward heazlewoodite (\left(\mathrm{Ni}{3} \mathrm{~S}{2}\right)), especially in alkaline solutions, the adsorption isotherm shape for dextrin on chalcocite (\left(\mathrm{Cu}{2} \mathrm{~S}\right)) indicates only low affinity. Our electrokinetic and coprecipitation measurements confirm such observations. The reta potential-ph curves for (\mathrm{Ni}{3} \mathrm{~S}{2}) and (\mathrm{Ni}(\mathrm{OH}){2}), and for (\mathrm{Cu}{2} \mathrm{~S}) and (\mathrm{Cu}(\mathrm{OH})_{2}) in the presence of dexIrin, exhibil quite different trends. While our results indicate strong interactions and flat orientation of dextrin macromolecules onto nickel hydroxide, and heazlewoodite in the (\mathrm{pH}) range close to the IEP of heazlewoodite, weak interactions between dextrin and chalcocite seem to result in an extended adsorption layer that drives the shear plane further away from the interface. 1993 Academic Press. Inc.