Micelle structure and composition has been determined by small-angle neutron scattering for mixed micellar solutions of in situ polymerized omega-methacryloyloxyundecyl-trimethylammonium bromide (MUTAB) in equilibrium with its monomeric form at various concentrations, as well as in mixtures with a fluorinated cationic surfactant, heptadecafluorodecylpyridinium chloride (HFDePC), and the non-ionic surfactant, C12E7. Whereas polymerized MUTAB is immiscible with HFDePC and forms two populations of distinct spheroidal micelles, it mixes with C12E7 in all proportions and forms a single average micelle structure depending on composition. These results allow us to explain the origin of the previously reported formation of mixed worm-like micelles of polymerized and monomeric MUTAB that coexist with globular monomeric MUTAB micelles as a consequence of the unfavourable electrostatic interactions that accompany the uncoiling of polymerized MUTAB chains in unimer micelles when swollen by monomeric cationic surfactants. (C) 2015 Elsevier Inc. All rights reserved.