Dynamic properties of quasi-binary aqueous solutions of ethyl(hydroxyethyl)cellulose (EHEC) and the hydrophobically modified analogue (HM-EHEC) have been investigated with the PFG-SE NMR technique. By taking advantage of the phase behavior of the (HM-)EHEC solutions, different polymer fractions could be obtained by simply adjusting the temperature within the two-phase region and collecting the coexisting phases. We discuss the connection between phase behavior, microphase structure, polymer structure, and polymer dynamics by following the self-diffusion of polymers in these fractions as a function of concentration and temperature. By extrapolating the diffusion coefficient to infinite dilution, we observe a significant difference in hydrodynamic radius between the separated fractions. The width of the distribution of diffusion coefficients is used to recognize differences in associative behavior of the different polymer fractions. The echo decays of highly associative fractions show, in addition to the characteristic curved echo decay commonly observed in solutions of associative polymers, a single diffusing component with a very slow diffusion coefficient (D ∼ 10-14 m2 s-1).