The association in aqueous solution of hydrophobically modified polyelectrolytes was investigated by F-19 and C-13 NMR. The polymers studied were poly(sodium acrylate) of average molecular weight ranging between 5 000 and 150 000 and bearing few mole percent of -CH2C7F15 or -C12H25 pendant alkyl chains. The F-19 and C-13 NMR spectra of these polymers display the characteristic features of slow exchange between the free and aggregated states for the alkyl chains. It is therefore possible to determine the respective fractions of free and aggregated side groups by direct integration of the NMR signals. Thanks to this determination the association mechanism and the dynamics of these systems were studied at a molecular level and compared to what is observed with classical surfactants.