Mixtures of the ionic surfactant sodium dodecyl sulfate (SDS) and nonionic surfactant dodecyl hexa(ethylene oxide) (C12E6) in heavy water were studied by small-angle neutron scattering. The ratio of SDS to C12E6 was varied under the constant total concentration of 25 mM and temperature of 15 degreesC. The formation of mixed micelles was observed. The structural and electrostatic properties of the mixed micelles change monotonically with varying surfactant ratio. Micelles become smaller and the degree of dissociation of SDS molecules in the micelles decreases when SDS is added. The observed behavior was compared with the results from light and neutron scattering studies of SDS/C12E6 in an aqueous saline solution (0.1 M NaCl, 25 degreesC) and with the predictions of molecular-thermodynamic theory.