We study the effect of electrostatic interactions in membranes by weakly charging swollen lamellar phases of several binary water-nonionic surfactant systems through the addition of ionic surfactants. For the water-C(12)E(5) system, preceding publications have presented evidence that undulations stabilize the lamellar phase with intermembrane spacings up to 300 nm. These undulations can be influenced by the addition of traces of ionic surfactants (1 ionic per 1000 nonionic molecules) leading to a substantial reduction of the spacings. The reduction can be understood as being caused by a flattening of the undulating membranes due to electrostatic repulsions of the ionic surfactant head groups. The bending elastic modulus of the purely nonionic membranes was previously determined to be of order k(B)T The electrostatic contributions to the bending energy are of comparative magnitude. Depending on the amount of ionic surfactant, values of up to 2k(B)T are observed. The experimental data are in sufficient agreement with theoretical predictions (Harden et al. Langmuir 1992, 8, 1170). For the binary water-C(12)G(2) and water-C(14)DMPO systems, similar observations are made. As the ionic concentration is increased for each system, a transition to an isotropic vesicle phase is induced. Freeze fracture electron microscopy illustrates for the water-C(12)E(3) system the nature of the vesicles.