We present a simulation study of the adsorption mechanism of water in a realistic carbon structure. Water molecules are modeled using a recently developed fixed-point charge water model optimized to the vapor-liquid coexistence properties.(1) Reverse Monte Carlo techniques(2) are used to generate a realistic porous carbon model composed of graphitic microcrystals consisting of rigid basal plates. Arrangements of the carbon plates are driven by a systematic refinement of simulated carbon-carbon radial distribution functions to match experimentally measured radial distribution functions. The adsorption of water in activated (having oxygenated surface groups) and nonactivated (graphitic) carbon is investigated using the grand canonical Monte Carlo simulation method. The adsorption behavior is found to be strongly dependent on the presence of activated sites. No appreciable adsorption occurs in the graphitic carbon until the pressure approaches the bulk gas saturation pressure, Effects of the surface site density, surface site position, and the magnitude of the surface site electrostatic charge on the adsorption behavior are investigated. Water adsorption is found to increase as the activated site density increases. The presence of water vapor in porous carbons is shown to dramatically affect the connectivity of the available pore space. Finally, cluster calculations are performed to investigate the growth mechanism of water clusters in activated carbons.