The effect of KCl on the adsorption of phenol, toluene, and benzene on activated carbon, with different degrees of surface oxygenation, was investigated. Different trends of salt effect were observed for each compound. The observed KCl effects were interpreted on the basis of electrical charge neutralization on the carbon surface and the adsorbate molecules, water adsorption, and the “salt out” effect. In particular, water adsorption was found to be crucial in reducing the adsorption capacity of the activated carbon. The influence of water adsorption was more pronounced on carbon surfaces with higher amounts of oxygen-containing groups. This was additionally confirmed by measurements using flow microcalorimetry.