The complexes formed between poly(ethylene oxide)-b-poly(ethylene imine)s and dodecanoic acid were studied in the solid state and as nanoparticles. The poly(ethylene imine) blocks had different architectures, cyclic, linear, and branched, while the poly(ethylene oxide) blocks were held constant. Self-organized lamellar-within-lamellar structures were obtained with two different length scales, which are, for example, 15 and 3 nm. Small-angle X-ray scattering methods were used to quantify the morphological differences between noncomplexed and complexed diblock copolymers. Core-shell nanoparticles were prepared from the complexes in aqueous solution with sizes around 200 nm. Their cores are formed by poly(ethylene imine) dodecanoate while their shells consist of poly(ethylene oxide). It was found that the shapes of the nanoparticles depend on the PEI block. They are, for example, prolate if the PEI is linear and spherical if the PEI is branched. The nanoparticles are block ionomer complexes and supposed to be potentially useful as drug carrier systems.