A sample of Westvaco carbon was oxidized with ammonium persulfate and both initial and oxidized samples were washed with methanol. The samples were characterized using sorption of nitrogen, Boehm titration, potentiometric titration, IGC, FTIR, TPD, and thermal analysis. Then water adsorption isotherms were measured at various temperatures close to ambient (relative pressure from 0.001 to 0.3). The isotherms were analyzed in terms of the thermodynamic consistency, presence of equilibrium, and water-activated carbon surface interactions. The results showed that washing with methanol significantly modifies the surface chemistry of carbons, creating phenols and new, very easily hydrolyzed esters. The latter, when in contact with water, is hydrolyzed, creating carboxylic acids and methanol. This may result in unjustified application of the Clausius-Clapeyron equation to calculate the isosteric heat of adsorption.