The water solubility of nonylphenol (NP) has been estimated to be 4.9 +/- 0.4 mg/l corresponding to (2.22 +/- 0.18) x 10(-5) mol/l at 25 degrees C using shake flask and surface tension techniques. The low solubility in combination with an observed rather slow dissolution process will limit the leachability of NP in the terrestrial environment. Based on indirect evidence, it is suggested that NP, in contrast to nonylphenol ethoxylate (NPEO) with, e.g., 12 ethoxylate moieties, is not subject to micelle formation, and as such does not constitute a potential vehicle for the transport of hydrophobic pollutants in the environment. For NPEOs with a very high number of ethoxy moieties, e.g., 100, the compounds appear water soluble without micelle formation.