In this study, a series of poly(L-lactic acid) (PLLA)/poly(ethylene oxide) (PEO) blends with different PLLA concentrations was prepared. Films of these blends crystallized with and without a cover were characterized by the presence and absence of banded structures, respectively. This difference in morphology could be observed because the PEO component of the blend was oxidized in air at high temperature (125 oC)without the protection of the cover. X-ray photoelectron spectroscopy (XPS) results showed that the surface of the blend crystallized without a cover contained mostly PLLA while the surface of the same blend crystallized with a cover slip contained only a moderately higher concentration of PLLA compared with the bulk. The migration of low molecular weight PLLA to the surface of the blends when no cover was used was due its low surface tension. Phase images obtained using atomic force microscopy (AFM) indicated that the banded structures consisted of valleys and ridges, which were in fact flat-on and edge-on lamellae, respectively. A detailed time-of-flight secondary ion mass spectrometry (ToF-SIMS) examination showed that PLLA and PEO were located mainly on the surfaces of the ridges and valleys, respectively. Acknowledgement The work described in this paper was fully supported by the Research Grants Council of the Hong Kong Special Administrative Region, China (grant No. 600210).