Amphiphilic block copolymers have attracted interest in recent years as drug delivery vehicles because of their capacity to encapsulate lipophilic drugs, medical imaging and diagnostic agents. In aqueous media, the block copolymers self-assemble into micelles (in thermodynamic equilibrium) or nanoparticles (kinetically-arrested), consisting of a hydrophobic core stabilized by a hydrophilic shell. Here we discuss the formation of nanoparticles from poly(ethylene glycol)-b-polylactone block copolymers for use as drug delivery vehicles. To determine the effect of the structure of the core-forming block on the loading and release of the solute, we have studied two different polylactone blocks: poly(e-caprolactone) which is semicrystalline, and poly(4-methyl-e-caprolactone) which is amorphous.