Abstract: Shengli lignite has been acid-washed. The chemical composition of ash from the raw sample was determined by X-ray Fluorescence (XRF). The acid-washed lignite was subsequently exchanged with different concentrations of Ca(CH₂COOH)₂ solution. Ion-exchange process was characterized by FT-IR, the ash content and pH value. The re-adsorption water contents of treated and raw samples were determined across a range of relative vapor pressures. The extend of ion-exchange reaction between Ca(CH₃COO)₂ and -COOH groups was controlled by the concentration of Ca(CH₂COOH)₂ solution. The effects of Ca²⁺ on equilibrium re-adsorption water values varied depending on relative vapor pressures and the amount of loaded Ca²⁺. Furthermore, the factor that controls equilibrium re-adsorption water content at relative vapor pressure of 92% or less is capillary force between Ca²⁺-carboxyl group complex and capillary. However, at higher relative vapor pressure the key factor becomes interactions of free water molecules.