Biodegradable amphiphilic block copolymers have potential applications in the fields of drug delivery and biomolecule sensing. Poly(ethylene oxide) (PEO), which is widely used to suppress the immune response to foreign materials, has many desirable properties as a stabilizing corona; however, block copolymer systems stabilized by PEO are often plagued by the formation of large inter-micelle aggregates. Our focus has been on understanding poly(ethylene oxide)-b-poly(caprolactone) (PEO-b-PCL) as a material for biomedical applications. In this work, the colloidal instability of primary spherical micelles of PEO45-b-PCL26 is discussed. Transmission electron microscopy and light scattering data suggest that primary spheres coexist with and slowly transform into elongated rodlike aggregates of spheres when suspended in deionized water at room temperature. The aggregation was found to be strongly affected by chemical alterations to the chain end of the PCL block. Acetylation of the terminal alcohol group had an effect on the kinetics of micelle aggregation, suggesting that the polarity of the chain end contributes to colloidal instability. End-capping with bulky hydrophobic groups seems to prevent the formation of these rods. Aggregated spheres were dispersed efficiently by heating, but the rods reformed on standing at ambient temperature. When heated above 600C, spherical micelles remained dispersed for a considerable period before rod formation was seen at rates similar to those seen at lower temperatures. Mechanisms of colloidal stability and aggregation of spherical micelles are presented with reference to their thermal behavior and the core end group effect.