In this work, the flotation of slime-fine fractions of Mt. Keith pentlandite ore was studied in de-ionised water, and bore water with high ionic strength. Compared with de-ionised water, bore water increased pentlandite flotation significantly while decreasing serpentine flotation. Carboxymethyl cellulose (CMC) was tested as a dispersant to improve pentlandite flotation in both de-ionised and bore water. The degree of substitute (DS) of CMC, an indication of the charge density, was found to be an important parameter. In de-ionised water, the higher the DS of CMC, the better the pentlandite flotation, while in bore water, the lower the DS of CMC, the better the pentlandite flotation. (C) 2011 Elsevier Ltd. All rights reserved.