Four coal samples of different rank were fully characterized with gas (N2 and CO2) sorption and FTIR methods. Then measurements of both equilibrium and kinetics of water vapor adsorption were undertaken at elevated temperatures. Equilibrium and kinetic data were fitted by the modified BET model and the unipore model, respectively. The corresponding thermodynamics, as well as the diffusion activation energy were estimated. The results demonstrate that water vapor adsorption depends on surface chemistry but unrelated to pore structure, because the pore space is not completely filled by water molecules even at saturation pressure. The monolayer capacity decreases with increasing coal rank. Moreover, water vapor needs lower work to attain equilibrium on higher-ranked coal. Also, the binding affinity of water molecule with primary sites is stronger than secondary sites. Furthermore, the diffusion coefficient decreases with coal rank, and the diffusion activation energy for high-volatile bituminous is the highest.