A novel system is described for the determination of the air-water partition coefficient (K-Aw) for substances that have low air-water partition coefficients, i.e., K-Aw < 10(-3), and may aggregate in solution, ionize, and display surface activity. The compound is evaporated isothermally from solution through an undisturbed air-water interface at a known gas flow rate, and its concentrations in the water and gas phases are measured. Although equilibrium is not achieved, the extent of departure from equilibrium can be determined using estimated mass transfer coefficients. KAw was determined for formic, acetic, benzoic, and perfluorooctanoic acids (PFOA), and assuming an approximately 50 % approach to equilibrium, which is in accord with theoretical prediction. Agreement with available literature data was satisfactory. The experimentally determined KAw of PFOA was 1.02.10(-3) with a standard deviation of 9.1 % (n = 9). The method is suitable for fluorinated surfactants, aggregating and ionizing substances for which KAw may not be readily measured with existing techniques.