We studied aqueous micellar solutions of mixtures of the nonionic surfactant octaethylene glycol mono-n-tetradecyl ether, C(14)E(8), and the cationic surfactant tetradecyl trimethylammoniumbromide, TTAB, through small angle neutron scattering. The mixed micelles maintained their spherical shape without effects of the charged molecules over the whole mixing range, from pure nonionic surfactant to pure cationic surfactant micelles. At very low composition (alpha = 0.04) of the charged TTAB molecules, the nonionic-cationic mixed micelles have a maximum aggregation number of the micelles. This aggregation phenomenon is described well by molecular-thermodynamic theory. There was synergistic interaction between the surfactants, leading to a maximum aggregation number of the micelles. This study presents an example of a spherical micelle for the synergistic interaction in nonionic-ionic surfactant mixtures. (C) 2011 Elsevier B.V. All rights reserved.