Measurements of equilibrium water contents (EWCs) in samples obtained from hydrothermal treatment (HT) and HT coupled with mechanical compression (HT–MC) were undertaken across a range of relative humidities (RHs) to investigate the performance of EWC and its mechanism. The changes in the concentrations of carboxyl groups and mesopore volume were measured by an improved barium ion exchange and N2 adsorption–desorption isotherms methods, respectively. The results showed that EWCs decreased with progressively severe HT and HT–MC conditions and EWCs of HT–MC samples were lower than those of HT samples, indicating that HT and HT–MC can upgrade lignite by reducing water loading capacity and HT–MC was better than HT. At low RHs (RH ≤ 10%), the factor that controls EWC is water molecules–active sites interactions and one to two water molecules are associated with each carboxyl group, while at medium RHs (10 < RH ≤ 92%) the amount of monolayer water and mesopore volume gain in significance and ca. two multilayer water molecules are bound to each monolayer water molecule. At high RHs (RH > 92%), EWC is determined by comprehensive factors such as macropores and cracks. Furthermore, EWC can be either higher or lower than residual water content (RWC) based on the RH within a threshold residual water level (ca. 5 to 16%). EWC was generally higher than RWC below a RWC of ca. 5% and the opposite relationship was observed for samples with RWC above ca. 16%. These provide information for the operation of lignite dewatering technique, the control of its water re-adsorption, and storage.