We report on the effects on the phase behavior in the water-rich part of the ternary nonionic surfactant system, pentaethylene glycol dodecyl ether (C(12)E(5))-water-decane, when adding sodium polyacrylate (PA) or hydrophobically modified polyacrylate (HMPA). The polyelectrolytes, having the same degree of polymerization, were derived from polyacrylic acid of M(w) = 150 000. The HMPA contains hydrophobic octadecyl side chains randomly anchored to the polyelectrolyte backbone, with a degree of modification of either 1 or 3% of the monomer units. The PA is found to be insoluble in the lamellar liquid crystalline and the liquid L(3) phases, containing infinite bilayer aggregates, while it is soluble in the liquid L(1) phase containing oil-swollen normal micelles. The HMPA, on the other hand, can be solubilized in all three phases, however, strongly affecting the phase equilibria. From the difference it is concluded that while PA is nonadsorbing, HMPA adsorbs to the surfactant film by the hydrophobic "stickers". In the presence of HMPA the surfactant films behave as effectively charged with long-range electrostatic interactions and a positive shift of the spontaneous mean curvature. The lamellar phase containing the HMPA phase separates at higher bilayer concentrations into two lamellar phases in equilibrium. Here a HMPA-rich lamellar phase with larger water spacings coexists with a lamellar phase of smaller water spacings essentially free of HMPA.