Well-defined mixtures of didodecyldimethylammonium bromide (DDAB), with either 12-dodecyltrimethylammonium bromide (DTAB), sodium n-dodecyl sulfate (SDS), or pentaethylene glycol monododecyl ether (C12E5), have been used to stabilize water-in-heptane microemulsions. Using DTAB or C12E5 with DDAB causes a synergistic effect, and the water solubilization capacity of the microemulsion is increased. However, with SDS there is a destabilization, indicating surfactant antagonism. Structure and dynamics in these systems have been studied as a function of surfactant type and composition using small-angle neutron scattering (SANS) and pulsed-field gradient NMR (PFG-NMR). Compositions of mixed films at these oil-water interfaces have also been measured, by blending deuterated DDAB with proteated DTAB or C12E5, and carrying out contrast variation SANS experiments. For both the DDAB/DTAB and DDAB/C12E5 mixtures, all the added surfactant was strongly adsorbed and there was no evidence for any significant partitioning into the solvent phases. These results show that changes in microemulsion phase behavior and properties are a direct consequence of preferred film curvature, which may be varied by surfactant mixing in the interface.