We have studied the effects of an adsorbing polyelectrolyte (hydrophobically modified sodium polyacrylate) on the phase behavior of a balanced nonionic surfactant-water-oil system. As seen from the variation of phase volumes in a three-phase (Winsor III) equilibrium, the effect of polymer adsorption can be described as an increase of the spontaneous curvature of the film away from water. With increasing polymer concentration the middle phase microemulsion first increases its water-to-oil ratio, whereas above a certain polymer concentration the trend is reversed. Following Kabalnov(1) et al., this reversal is interpreted as a saturation in the polymer adsorbtion. The saturation value corresponds to approximately 0.2 mg of polymer per m(2) of the surfactant film and is similar to that in the Kabalnov experiment with a different polymer. The phase behavior at equal volumes of water and oil was also studied as a function of surfactant concentration for various aqueous polymer concentrations. While the polymer is readily soluble in the microemulsion phase with bicontinuous topology, its solubility in the lamellar phase, where the surfactant (bilayer) film is planar. was found to depend on the polymer concentration and added salt. The results demonstrate that film topology and electrostatic interactions are important factors determining the polymer compatibility with these surfactant phases.