PEG derivatives of molecular weights of approximately 5K and 10K and end-functionalized with C6F13 groups in aqueous solutions show strong association of the perfluorocarbon end groups into dimers as indicated by F-19 NMR and reduced viscosity studies. In sharp contrast, the C8F17 derivatives in aqueous media appear to associate into micelles. Association of the C6F13 groups is favored by a lower PEG molecular weight, presumably due to excluded volume interactions. Addition of NaCl and increasing temperature favors association of the 5K C6F13 derivative, consistent with hydrophobic association. Increasing temperature also promotes hydrophobic association into dimers as indicated by a positive change in the entropy of association, Delta S, of 21 cal mol(-1) K-1 and a relatively large positive value of the change in the enthalpy of association, Delta H, of 3.5 kcal/mol. Viscometric studies appear to be consistent with the fluorine-19 NMR studies but additionally suggest the occurrence of hydrophobic interactions of the perfluorocarbon groups with the PEG surfactants.