A set of PEG-b-PBO-b-PCL triblock copolymers were prepared for comparative studies of micelle formation and relative stability compared to the parent PEG-b-PCL diblock copolymers. Block copolymers that were characterized by (1)H NMR and GPC were self-assembled in water by nano-precipitation from organic solvents. Spherical micelles produced in near quantitative yield were characterized by dynamic light scattering (DLS) and transmission electron microscopy (TEM). The initial step in degradation of the block copolymers assembled in spherical micelles is shown by GPC and (1)H NMR in acidic media to occur by hydrolysis of the interface ester group which cleaves the PCL segment in the core from the -PBOPEG and -PEG segments. Kinetics of hydrolysis of the ester groups that bind the hydrophobic PCL segment with either PEG or PBO units were followed by (1)H NMR and demonstrate that the triblock has a reduced rate of hydrolysis. Inserting a short block of PBO between the PEG and PCL segments provides increased protection from hydrolysis for the ester group at the hydrophilic-hydrophobic interface.