Mg ion-exchanged samples were prepared with acid-washed Shengli lignite. The chemical composition of the ash of the raw sample was determined by X-ray fluorescence. The equilibrium adsorption water contents of samples were determined in a range of relative humidity. The ion-exchange process was characterized by FT-IR, ash content, and pH value. A possible mechanism is proposed for equilibrium adsorption water of ion-exchanged samples at different humidities. The extent of ion-exchange reaction between Mg2+ and lignite is controlled by the concentration of Mg2+ in MgSO4 solution. The effect of Mg2+ on equilibrium adsorption water content varies with relative humidity and content of Mg2+. The factor that controls equilibriuma dsorption water content at low relative humidity is water interactions with sorption sites, which are Mg2+-carboxyl group complex. At middle relative humidity capillary force between Mg2+-water clusters Mg+(H2O)-H-()(n) and capillary is more important. At high relative humidity, free water-free water interactions are more significant. (C) 2014 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.