In aqueous solution, it was found that the amphiphilic copolymer poly(ethylene glycol)-b-poly(caprolactone) (PEG(5000)-b-PCL4100) formed different morphologies, including long rod-like, short rod-like, or spherical aggregates, when the copolymer concentration was increased. Nearly identical morphologies were observed with the addition of increasing amounts of PEG(2000)-distearoylphosphoethanolamine (PEG(2000)-DSPE) to the copolymer. The morphologies of the aggregates in solution were confirmed by negative stain transmission electron microscopy (TEM) and cryogenic-TEM (cryo-TEM). The critical micelle concentrations of the PEG(5000)-b-PCL4100 copolymer, PEG(2000)-DSPE and a mixture of the two materials (PEG(5000)-b-PCL4100/PEG(2000)-DSPE) were evaluated to determine the thermodynamic stability of the aggregates. Differential scanning calorimetry was performed to gain insight into the degree of mixing of PEG(5000)-b-PCL4100 and PEG(2000)-DSPE. Overall, combining PEG(5000)-b-PCL4100 and PEG(2000)-DSPE produced a single population of mixed micelles with rod-like or spherical morphologies depending on the material composition and concentration.