This study aims to understand the fundamental mechanism underpinning the different behaviour of ionic dispersants in fresh water and saline water observed in coal and mineral flotation and many other disciplines dealing with particles. Lignosulfonate D748 (LS), an anionic dispersant was used in this study to mitigate the clay slime coating in coal flotation using deionised water, and saline water of high ionic strength. In deionised water, the addition of a small amount of lignosulfonate enhanced coal flotation in the presence of clay minerals, but this beneficial effect was not observed in saline water. The addition of a large amount of lignosulfonate depressed coal flotation in both deionised water and saline water. The underlying mechanism was investigated by a range of techniques including flotation of pure coal, adsorption tests, and atomic force measurements (AFM). It was found that the enhanced electrostatic repulsion induced by a small amount of lignosulfonate in deionised water was responsible for the mitigation of clay coatings on the coal surface, resulting in improved coal flotation. In saline water, a low amount of lignosulfonate could not disperse clay minerals from the coal surface due to insufficient steric repulsion, and flotation improvement was therefore unable to achieve. The depression of coal flotation at a high lignosulfonate concentration in deionised water and saline water was attributed to the high adsorption on the coal surface, rendering it strongly hydrophilic regardless of the removal of clay coatings. (C) 2014 Elsevier Ltd. All rights reserved.