The major loss of PGM and base metals in beneficiation of the Merensky Reef ore occurs during the separation of the siliceous gangue from the base metal sulphides and PGM minerals by selective flotation. The predominant gangue minerals in Merensky ores fed to the PGM concentrators are pyroxene and feldspar. Other important gangue minerals are talc and chlorite. A significant percentage of these minerals reports to the concentrate, thus diluting the concentrate grade and ultimately increasing the transport and smelting costs. Feldspar is naturally hydrophilic and requires activation for flotation. The present work investigates how collector adsorption (sodium isobutyl xanthate), copper sulphate activation, and the distribution of ions on mineral surfaces may influence selectivity in the flotation of these ores. The possible chemical reactions taking place on the surface of synthetic pentlandite, natural feldspar, and a 1:1 mixture were investigated. In the pH range studied, zeta potential and time of flight secondary ion mass spectrometry (ToF-SIMS) analyses indicated the presence of xanthate and copper ions on pentlandite surfaces. Xanthate ions did not adsorb onto feldspar surfaces. However, at pH 9, feldspar surfaces became coated with copper species and the subsequent adsorption of xanthate ions caused the feldspar to float. The results are explained in terms of the interaction between various copper and xanthate species present and the zeta potentials of the minerals at the pH values of interest. (C) 2002 Elsevier Science B.V. All rights reserved.