We have investigated the association of hydrophobically modified poly(sodium acrylate) (HMPA) to micelles and bilayers of nonionic surfactant. The HMPA bears octadecyl chains randomly anchored on 1 or 3 mol % of the monomer units, and the surfactant is pentaethylene glycol dodecycl ether (C(12)E(5)) or octaethylene glycol dodecyl ether (C(12)E(8)). The HMPA was introduced in small amounts to the micellar and lamellar phases of the binary nonionic surfactant/water system. The association of HMPA with the nonionic surfactant aggregate is due to the hydrophobic alkyl chains of the polymer that dissolve in the surfactant film. This association behavior has a large impact on the theological properties of the aqueous HMPA-nonionic micelles mixtures. Moreover, the bonding of the surfactant on the polymer induces a pronounced decrease of the micellar mobility as is evident from molecular self-diffusion measurements. Small amounts of HMPA dramatically increase the stability of the micellar phase. The change in phase behavior is similar to that obtained by adding small amounts of ionic surfactant to the surfactant film. Adding HMPA to the water/C(12)E(5) lamellar phase results in a strong decrease in turbidity, indicating a crossover from a dominating undulation to a dominating electrostatic repulsion between bilayers.