A poly(2-ethylhexyl acrylate)-block-poly(methyl methacrylate)-block-poly(acrylic acid) (PEHA-PMMA-PAAc) triblock copolymer was prepared by group transfer polymerization (GTP) and subsequent acidolysis. When transferred from a molecular solution into water or D2O, the copolymer is shown, by static light scattering (SLS) and dynamic light scattering (DLS), to form spherical micelles of a narrow size distribution. According to NMR line shape and signal intensity analysis, and SANS measurements using contrast variation by a selective swelling of PEHA blocks with cyclohexane-d(12), the three block types are mostly segregated in a micelle, PEHA forming the inner and PMMA the outer layer of the core. Both methods indicate an existence of mixed layers in which the blocks PEHA-PMMA (in the core) and PMMA-PAAc (at the outer rim of the core) are entangled. NMR data point to the temperature-dependent conflict of the influence between the PEI-IA compression resistance and the core-shell interface tension. Solvents generally penetrate more slowly into PEHA-PMMA-PAAc micelles than into PMMA-PAAc micelles of a comparable size, but those selective for PEHA, such as cyclohexane, swell the micellar core to a higher degree in the former case.