Thermodynamic expressions for the free energy of contact angle hysteresis have been derived assuming that wetting can be modeled as an adsorption-desorption process. The cause of hysteresis is not specified; it could be due to surface imperfections or simply irreversible interaction of the contact liquid and solid. The model shows that even near-zero contact angles can represent substantial free energies. Experimental data from previous work were used to calculate free energies for a variety of polymer and surfactant surfaces. For each surface, a variety of organic liquids and water gave similar free energy values. Nonpolar polymer surfaces gave free energies that corresponded to van der Waals interactions. Free energies were usually higher for polar polymers, arising from stronger interactions that may involve more extensive restructuring of groups at the interface.