Abstract: To investigate the effects of cations on equilibrium re-adsorption water content of lignite, the acid-washed Shengli lignite was exchanged with different concentrations of K₂SO₄, Na₂SO₄, MgSO₄, Ca(CH₂COOH)₂ solution, respectively. Ion-exchange process was characterized by the changes in wavenumber of carboxyl group and the ash contents. The equilibrium adsorption water contents of all samples were determined in a range of relative humidities. It is shown that the extent of ion-exchanged exchange in Shengli coal is in order of Ca²⁺>Na⁺>K⁺>Mg²⁺. The relative effectiveness of cations in increasing the equilibrium adsorption water content is in order of Mg²⁺>Ca²⁺>Na⁺≈K⁺. At high relative humidity, the factor that control equilibrium adsorption water content is free water molecules interactions. However, at middle relative humidity capillary force between metal cation-carboxyl group complex and capillary is more important. At low relative humidity water interactions with sorption sites which are cation-carboxyl group complex become more important.