We have studied the effects on microstructure and phase equilibria of a nonionic surfactant-water-oil system when replacing one per hundred of the surfactant molecules by ionic surfactant, sodium dodecyl sulfate (SDS). This degree of substitution corresponds to introducing approximately one charge per 4400 Angstrom(2). The nonionic surfactant system contains pentaethylene glycol dodecyl ether (C(12)E(5)), water, and decane, at a constant surfactant to oil weight ratio of 52/48. The water content is varied from 60 to 100 wt %. In this composition range the initial phase sequence consisted of L(1) phase-lamellar phase-L(3) phase as a function of increasing temperature. Adding the small amount of SDS has a large impact on the phase behavior. A reverse hexagonal phase (H-2) and a reverse micellar microemulsion phase (L(2)) are added to the phase sequence at temperatures above the L(3) phase. The microstructures in the different phases were studied by the NMR self-diffusion technique and by small angle X-ray scattering.