A simple and precise method for determining the Henry's law constant for hydrophobic organic chemicals was developed. Henry's law constants were obtained as air-water partition coefficients in a dilute solution from the two partition coefficients between polydimethylsiloxane (PDMS) and air (K-PDMSa,) and between PDMS and water (K-PDMSw). Measurements of the low concentrations in air or water at phase equilibrium were avoided. Instead, the K-PDMSa and K-PDMSw,, values were obtained by measuring the mass transfer rate constants in a boundary layer and relating them with K-PDMSa, and K-PDMSw using a film diffusion model. Twenty hydrophobic chemicals (11 polycyclic aromatic hydrocarbons, 5 chlorinated benzenes, 2 phthalate esters, and 2. aromatic nitro musk;) with literature values of the Henry's law constant ranging from 10(-2) 10(2) Pa.m(3).mol(-1) were chosen to evaluate the method. The Henry's law constants derived in this study agreed very well with the experimentally determined values from the literature. Because the proposed method provides a fast and simple method for measuring the Henry's law constant within three days, it is a very promising method for generating the Henry's law constants widely used in the assessment of the environmental fate of hydrophobic chemicals.