Neutron reflectivity of lamellar phases containing pentaethylene glycol n-dodecyl ether, sodium decylsulfonate, and D2O is measured at the solid-liquid interface. The scattering length density profiles deduced from fits to the reflectivity data show that undulations of the mixed surfactant bilayers close to the solid-liquid interface decrease as the molar ratio of ionic to nonionic surfactant increases. This is consistent with recent theoretical predictions. Small angle neutron scattering measurements of these samples show both small angle scattering and quasi-Bragg peaks. As the molar ratio of ionic to nonionic surfactant increases, the low-q scattering intensity decreases and higher order quasi-Bragg peaks appear. These changes in the bulk neutron scattering signal are consistent with the changes observed with reflectivity.