If precautions are taken to eliminate or account for nonsettling (or nonfilterable) microparticles or organic macromolecules that remain in the aqueous phase during laboratory sorption tests, the observed partition coefficients (Kp or Koc) for a group of model hydrophobic organic compounds (PCBs) are found to remain constant over a wide range of solid-to-solution ratios. Further, the partition coefficients for either sorptive uptake or desorptive release are indistinguishable and confirm the reversible nature of hydrophobic sorption. It is proposed that descriptions of the 'speciation' of hydrophobic compounds in natural waters should include not only dissolved and sorbed-to-sediment fractions but also a component sorbed to nonsettling microparticles or organic macromolecules.