Mg ion-exchanged samples were prepared with acid-washed Shengli lignite. The chemical composition of the ash of the rawsamplewas determined by X-ray fluorescence. The equilibriumadsorptionwater 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 Mg

2+

and lignite is controlled by the concentration of Mg

2+

in MgSO

4

solution. The effect of Mg

2+

on equilibrium adsorption water content varies with relative humidity and content ofMg

2+

. The factor that controls equilibriumadsorptionwater content at low relative humidity is water interactions with sorption sites

which are Mg

2+

-carboxyl group complex. At middle relative humidity capillary force between Mg

2+

-water clustersMg+(H

2

O)

n

and capillary is more important. At high relative humidity

free water-free water interactions are more significant.