The phenomenon of water adsorption in microporous carbons is investigated using activated carbon fibers as the adsorbents. It is shown that the process of water adsorption is due to both physical adsorption and chemical interaction with surface groups. In accordance with this process, the filling of the micropores is shown to be progressive, the narrower micropores being filled first, and then water is adsorbed in the whole range of microporosity. This study confirms that water adsorbed in the porosity of the carbon has a solid-phase structure throughout the whole range of micropore size and, therefore, the density of adsorbed water has to be considered to be around 0.92 g cm(-3).