We analyze in detail the morphology transition of poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEO-b-PNIPAM) diblock copolymer in water with small angle neutron scattering. A three-level hierarchical structure is found below the critical solution temperature (LCST) of PNIPAM. The small-scale structure corresponds to the unperturbed PEO-b-PNIPAM chains. These coils assemble into intermediate-size aggregates which have a constant number density in solution, independently of the solution concentration, under the assumptions of a simple model based on the concentration dependence of the fitting parameters. Large clusters are also observed, which comprise the large-size structure. Above the LCST the system transforms into a well-defined geometry of monodisperse elliptical vesicles which does not depend on the solution concentration. The vesicles become less asymmetric with a higher aggregation number at the highest temperature. (C) 2013 Elsevier Ltd. All rights reserved.