Breakthrough behavior is investigated for an activated carbon bed fed with constant concentrations of organic vapors and a stepped concentration of water vapor. The organics are present as single components and binary mixtures, with both hydrophobic and hydrophilic organics considered. Experiments are performed using a fully automated fixed-bed apparatus, which enables breakthrough curve studies for gas mixtures over a wide range of concentrations, from much less than 1 ppm up to saturation. The adsorbed-phase mass transfer coefficient of water is found to be a strong function of water loading. The system response is complex. with behaviors for hydrophilic and hydrophobic compounds being quite different. A model incorporating all accurate description of the highly nonideal adsorption equilibria is developed to simulate all of the fixed-bed experiments and gives good agreement in time, trend, extent, and shape of the breakthrough curves.