An investigation was carried out on the adsorption and desorption of moisture in chars of low-rank coal. Equilibrium moisture sorptions of dry and moist chars were measured at room temperature and at a relative humidity of 30% and 80%. On the basis of these measurements, a simple mathematical model was developed to predict both the rate and the level of hydration for coals and chars. The formulation uses a shrinking core model which required only the measurement of the adsorbing material＇s equilibrium moisture content at different temperatures and humidities. The model was validated against experimental and literature data. It accurately and reliably predicted both the rate and extent of hydration and dehydration for coals and char. Using this model, the effects of varying temperatures, relative humidities, and size of the particles and coal pile were simulated. The sensitivity study demonstrated that, as expected, relative humidity and temperature had strong effects on both the rate of hydration and the equilibrium moisture of coal or char. The particle size dramatically influenced the rate of hydration but had no affect on the equilibrium moisture content. This model can be used effectively to simulate the impact of moisture on drying, storage, and spontaneous combustion of coals and coal-derived chars.