The paper presents the results of viscosity measurements by capillary and rotational viscometry for aqueous solutions of sodium dodecyl sulfate and dodecyldimethylamine oxide and data on the hydrodynamic radii of aggregates obtained from dynamic light scattering measurements. A wide range of solution compositions predominantly containing dodecyldimethylamine oxide was studied at 308.15 K; the total content of surfactants was from 1 to 15 wt %, pH ∼ 8. Solution viscosities were found to change by several orders of magnitude when the composition of the system was varied; the dependence of viscosity on the ratio of surfactants passes through maximum and inflections. The dependences of viscosity and hydrodynamic radii on the ratio between two surfactants were similar in character. The conclusion was drawn that various nanostructural rearrangements occurred in the solutions studied and synergistic effects were observed.