Activated carbon (AC) was modified by oxidation with nitric acid and microwave-assisted reduction. The textural properties and surface oxygen content as well as the surface acidities of the ACs studied were determined by nitrogen adsorption, X-ray photoelectron spectrometry, and Boehm titration. The adsorption isotherms of the water vapor were measured, and temperature-programmed desorption experiments were conducted to estimate the desorption activation energy, E(d), of water on the ACs. The results indicated that the oxidation increased the value of E(d) and the adsorption capacity of water on AC in low relative humidity (RH) but decreased the adsorption capacity of water in high RH. The reduction decreased the value of E(d) and the adsorption capacity of water on AC over the whole RH range. The modification of AC first by the oxidation treatment of nitric acid and then by microwave-assisted reduction treatment not only increased the adsorption capacity of water in a high RH but also greatly decreased the value of E(d). The microwave-assisted reduction of virgin or oxidized AC with hydrogen should yield a lower E(d) and higher adsorption capacity of water in high RH than the reduction of that with nitrogen.