The effects on the microstructure and phase equilibria of a nonionic surfactant/water/oil system when replacing one, four, and six per hundred of the nonionic surfactant molecules by the ionic surfactant sodium dodecyl sulfate (SDS) have been studied. The nonionic surfactant system consists of pentaethylene glycol dodecyl ether (C(12)E(5)), water, and decane, at a constant surfactant to oil weight ratio of 51.9/48.1. The phase equilibria and the corresponding microstructures were investigated as a function of temperature and water content. As a function of temperature, the initial phase sequence for this composition range was L(1) phase-lamellar phase-L(3) phase. Three effects on phase behavior are observed when introducing electrostatic interactions. (i) Water rich reverse hexagonal (H-2) and reverse micellar (L(2)) phases replace the two-phase region water + L(3) phase. (ii) The crystallization of micellar cubic (I-1) and normal, hexagonal phases moves to lower particle concentrations. (iii) Phase boundaries at higher water contents are strongly shifted to higher temperatures. The latter effect is analyzed in detail at the emulsification failure boundary where spherical oil-swollen micelles are in equilibrium with excess oil. Using the bending energy approach together with Poisson-Boltzmann calculations within the cell model, we have analyzed the charge dependence of the emulsification failure boundary. The SDS effects can be reproduced with a cationic surfactant. We also show that the electrostatic effects are removed upon addition of salt. The microstructure in the various phases was studied by small angle X-ray scattering (SAXS). A neutral surface located at the polar/apolar interface is identified where the area per surfactant molecule is constant irrespective of the curvature.