Surfactant mixing in model water-in-heptane microemulsion interfaces has been investigated for blends of a cationic didodecyldimethylammonium bromide (DDAB) with poly(ethylene glycol) monododecyl ethers (C12EJ, J = 3, 4, 5, 6, 7, 8, and 23). Phase behavior studies, electrical conductivity, and contrast variation small-angle neutron scattering (SANS) have been employed to delineate the effects of systematic variation of ethylene oxide headgroup, size. These mixtures are characterized by an overall surfactant concentration (0.10 mol dm(-3)) and mole fraction of nonionic, which was varied up 0.20. The larger ethylene oxide (EO) numbers of 5-7 and 23 lead to significant enhancements in the maximum microemulsion solubilization capacity compared to DDAB only, whereas the shortest surfactant employed, C12E3, caused a decrease in the phase stability. Microemulsion nanostructure and interfacial compositions were studied for the EO3, EO4, EO6, and EO7 systems in partial structure factor type SANS experiments, as described before for the EO5 analogue (Langmuir 1999,15,5271). Analysis of contrast variation SANS data showed that C12E7, C12E6, and C12E5 partition strongly into the DDAB layer. Under equivalent conditions the shorter EO chain surfactants C12E4 and C12E3 appear to adsorb much more weakly. Interfacial compositions determined by SANS have been used to rationalize trends in phase behavior and nanostructure, highlighting the importance of partitioning effects with nonionics in multicomponent mixtures of this type.