Resonantly enhanced surface second harmonic generation has been used to measure the adsorption isotherm of dimethyl sulfoxide (DMSO) at the liquid/vapor interface of water. The Langmuir model of interfacial adsorption was used to model the data allowing determination of -11.8 (sigma = 1.2) kJ/mol for the Gibbs free energy of adsorption of DMSO to the liquid/vapor interface of water at 293 K. This value of the adsorption energy is combined with data obtained from the literature to yield a complete thermodynamic characterization of a model potential energy surface for the transport of DMSO across the liquid/vapor interface of water. Some implications of this potential energy surface on the kinetics of mass transport are presented.