The aggregation in aqueous solution of a hydrophobically modified polyelectrolyte was investigated by F-19 n.m.r. The polymer studied was a poly(sodium acrylate) of average molecular weight 5000 bearing 8 mol% of -CH2C7F15 pendant alkyl chains. The F-19 n.m.r. spectra of the modified polymer displayed the characteristic features of slow exchange between the free and the aggregated states for the alkyl chains. It is therefore possible to determine the respective fractions of free and aggregated -CH2C7F15 groups by direct integration of the n.m.r. signals. This behaviour is opposite to the fast exchange occurring for classical surfacants (sodium pentadecafluorooctanoate, SOF). The exchange process in the polymer case is at least three orders of magnitude slower than in the surfactant system.