The self-assembly of dialkyl chain cationic surfactant dihexadecyldimethyl ammonium bromide, DHDAB, and nonionic surfactants monododecyl hexaethylene glycol, C12E6, and monododecyl dodecaethylene glycol, C12E12, mixtures has been studied using predominantly small-angle neutron scattering, SANS. The scattering data have been used to produce a detailed phase diagram for the two surfactant mixtures and to quantify the microstructure in the different regions of the phase diagram. For cationic-surfactant-rich compositions, the microstructure is in the form of bilamellar, blv, or multilamellar, mlv, vesicles at low surfactant concentrations and is in an L-beta lamellar phase at higher surfactant concentrations. For nonionic-rich compositions, the microstructure is predominantly in the form of relatively small globular mixed surfactant micelles, L-1. At intermediate compositions, there is an extensive mixed (blv/mlv) L beta/L-1 region. Although broadly similar, in detail there are significant differences in the phase behavior of DHDAB/C12E6 and DHDAB/C12E12 as a result of the increasing curvature associated with C12B12 aggregates compared to that of C12E6 aggregates. For the DHDAB/C12E12 mixture, the mixed (blv/mlv) L-beta/L-1 phase region is more extensive. Furthermore, C12E12 has a greater impact upon the rigidity of the bilayer in the blv, mlv, and L regions than is the case for C12B6. The general features of the phase behavior are also reminiscent of that observed in phospholipid/surfactant mixtures and other related systems.