The influence of the nonionic polymer poly(N-vinyl-2-pyrrolidone) (PVP) in comparison to the surfactant 1-octyl-2-pyrrolidinone (OP) on the phase behavior of the system SDS/pentanol/xylene/water was studied. In both modified systems a strong increase in the water solubilization capacity was found, accompanied by a change in the spontaneous curvature toward zero. In the polymer-modified system an isotropic phase channel is formed with increasing polymer content that connects the Ll and the L2 phase. The lamellar liquid crystalline phase is destabilized in both cases. In the Ll phase the adsorption of PVP at the surface of the microemulsion droplets and the formation of a cluster-like structure is proven by several methods like C-13 NMR T-1 relaxation time measurments, zeta potential measurements, and rheology. In the L2 phase a modification of the interface of the inverse droplets is detected by a shift in the percolation boundary (conductivity) and C-13 NMR T-1 relaxation measurements. The formation of a cluster-like structure can be assumed on the basis of our rheological measurements.