Two polylactide-b-poly(perfluoropropylene oxide) (PLA-PFPO) diblock copolymers were synthesized via a coupling reaction of acid chloride end-functionalized poly(perfluoropropylene oxide) and hydroxyterminated polylactide. The solubility of these materials in CO 2 was measured using a variable-volume high-pressure cell. At a concentration of 1 wt %, the PLA-PFPO copolymers were found to be soluble at modest pressures (<500 bar) over the temperature range of 30-65°C. The size of the resulting micelles in solution was characterized by high-pressure dynamic light scattering. These measurements indicated the formation of predominantly small, spherical micelles for the block copolymer with a PFPO volume fraction of 0.49 and large aggregates with hydrodynamic radii of 100 nm for PLA-PFPO with a PFPO volume fraction of 0.37. By kinetically trapping the aggregates in CO 2 through vitrification of the PLA cores, redispersing them in an analogous PFPO selective solvent, and imaging them using transmission electron microscopy, we confirmed the formation of block copolymer vesicles in liquid CO 2.