This paper describes an optimized experimental system for determining water/polymer coating distribution constants and discusses the mechanism of extraction of high molecular weight compounds by poly(dimethylsiloxane) (PDMS). The proposed flow-through standard water generator eliminates errors associated with both losses of analytes to the system surfaces and limited sample volume. In addition, the errors caused by partial precipitation or poor dissolution of highly hydrophobic analytes during spiking of water when producing calibration standards are also eliminated. The target analytes are a range of polycyclic aromatic hydrocarbons (PAHs). The results demonstrate that absorption partitioning is the predominant mechanism of extraction of analytes into PDMS-coated fibers for all PAHs investigated regardless of molecular weight. There is a strong correlation between determined PDMS/water distribution constants (K(fw)) and literature K(ow) values for all PAHs.