With the use of the grand canonical Monte Carlo simulation, water adsorption isotherms were determined for SPC/E water in slit-shaped graphitic nanopores at 298 K. The pore widths considered were 0.6, 0.625, 0.7, 0.8, 1.0, 1.6, and 2.0 nm. The resulting adsorption isotherms indicated negligible adsorption at low pressures, pore-filling by a capillary-condensation-like mechanism, and adsorption/desorption hysteresis loops. For pore widths equal to or larger than 0.7 nm, the relative pressure at which pore filling occurs and the size of the hysteresis loop decrease with decreasing pore width. For 0.6-nm pores, pore filling occurs at pressures approaching saturation. Upon decreasing the pore width from 2.0 to 0.7 nm, the zero-coverage isosteric heat of adsorption increases from 6 to 14 kJ/mol. The limit at high coverage converges to the enthalpy of condensation for SPC/E water as the pore width increases. From the simulated adsorbed-water densities and distributions across the pores and the use of a mean-fie...