While mixtures of ionic polymers and ionic surfactants have been extensively studied, mixed systems of nonionic surfactants and nonionic amphiphilic copolymers have largely been ignored. Direct microstructure imaging by cryotransmission electron microscopy of aqueous mixtures of a nonionic surfactant C12E5 and two uncharged block copolymers-diblock copolymer poly(ethyleneoxide)-polybutadiene (EO126-B-45) and triblock copolymer poly(ethyleneoxide)-poly(ethylethylene)-polybutadiene (EO21-EE35-EO21)-revealed that the shapes and sizes of mixed micelles change as functions of the surfactant-to-copolymer concentration ratio. Cylindrical micelles of EO21-EE35-EO21 are transformed to spherical micelles upon the addition of C12E5, while large spherical micelles of EO126-B-45 change into smaller spheres when mixed with the nonionic surfactant. The range of surfactant-to-copolymer concentration ratios in which the micelle microstructure changes was correlated with the solution surface-tension variation. The triblock copolymer showed different effects than did the diblock counterpart on mixed micellar behavior. The nonionic mixed amphiphile system, with the near-monodispersity of components and the simple chemical structure, is a good candidate for theoretical modeling.