Water vapor adsorption/desorption isotherm studies were performed on high volatile bituminous B coal and lignite to investigate the behavior of water on coal. The characterization of water sorption was studied by kinetic and equilibrium data, at a temperature of 298 K and a relative vapor pressure tip to 0.95. Water sorption isotherms have a standard type 11 sigmoid shape obeying the BET model in the relative pressure range 0.05-0.35. A modified BET model was used to estimate the water adsorbed on primary and secondary sites, respectively. The specific surface areas estimated with water and Cot were similar and about 95 and 52 m(2) g(-1) for high volatile bituminous B coal and lignite, respectively. The adsorption capacity of water and CO2 was related to the oxygen content of coal and the proportion of inorganic matter. The water adsorption and desorption isotherms produce a different hysteresis loop for the two coals. On the other hand, the diffusion coefficient of water adsorption was estimated at about 10(-16) m(2) s(-1), by the unipore model. From the diffusion coefficient of water, which varied with increase in relative pressure, it is shown that water adsorbs first on primary sites, before formation of water clusters and their micropore filling which was a slow process. (C) 2009 Elsevier Inc. All rights reserved.