Small angle neutron scattering and light scattering measurements have been used to probe interactions between mixed micelles composed primarily of hexaethylene glycol mono-n-dodecyl ether, C12E6, with small amounts of added sodium dodecylsulfonate, C12SO3Na, in D2O. The phase behavior of these two surfactants in D2O varies little from their phase behavior in H2O. As the ratio of anionic to nonionic surfactant increases at fixed nonionic surfactant concentration, static light scattering spectra from micellar solutions show growing evidence of an interaction peak that correlates with expected increases in electrostatic repulsions. Dynamic light scattering results are also consistent with an increase in the repulsive interactions between micelles as the amount of added C12SO3Na increases. Small angle neutron scattering spectra are used to test models of micelle interactions. A model incorporating an electrostatic perturbation to the sticky hard sphere model provides excellent fits to spectra from mixed micellar solutions. The size of the micelle remains constant with added anionic surfactant, and the charge per micelle increases steadily. Thus, the effect of adding anionic surfactant to aqueous micellar solutions of C12E6 can be understood simply in terms of a strengthening repulsive electrostatic component of the micelle-micelle interaction potential that counteracts an attractive potential between C12E6 micelles.