Mixed solutions of two surfactants, dodecyldimethylamine oxide (C(12)DAO) and sodium dodecyl sulfate (SDS), were investigated by the small-angle neutron scattering (SANS) method. The SANS intensities were analyzed by using the prolate ellipsoidal model for the intraparticle structure factor and the rescaled mean spherical approximation (RMSA) for the interparticle structure factor. Prolate ellipsoidal micelles with axial ratios of 1.3-4.5 were formed in the mixed solutions. Average numbers of hydrated D2O per surfactant on micelles were 6-9. At equimolar mixing, micellar sizes became maximum and the D2O hydration minimum. The formation of ion pairs between protonated C(12)DAO and SDS is an origin of such nonideal behavior. Electric charge numbers per surfactant in micelles increased with adding SDS, indicating the constant charge number of 0.13-0.15 per SDS. The addition of NaCl shielded the electric charge, resulting in the reduction of the repulsive interaction between micelles.