The parameters which influence the self-assembly of molecules in solution include the temperature and solvent quality, and this study illustrates the use of these variables to regulate the degree of association of block copolymer amphiphiles in highly compressible supercritical carbon dioxide. Small-angle neutron scattering (SANS) has been used to examine the association behavior of a block copolymer containing a CO2-phobic moiety, poly(vinyl acetate), and a CO2-philic block, poly(1,1-dihydroperfluoro-octylacrylate). By adjustment of the density of the medium through pressure and temperature profiling, the self-assembly can be reversibly controlled from unimers to core−shell spherical micelles and this establishes a critical micelle density (CMD), a phenomenon distinctive of highly compressible fluids, such as supercritical CO2. Mathematical modeling of the data in terms of core−shell micelle structures permits a detailed description of the structure and the degree of swelling (penetration) of the solvent into the different regions of the aggregates throughout this transition.