The effect of aging on the fractional crystallization of the poly(ethylene oxide) (PEO) component in the PEO/poly(3-hydroxybutyrate) (PHB) blend has been investigated. The partial miscibility of the PEO/PHB blends with high PEO molecular weight (M-v = 2.0 x 10(5) g/mol) was confirmed by differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis. The fractional crystallization behavior of the PEO component in the PEO/PHB blends with low PEO content (not more than 30 wt% of PEO), before and after aging under vacuum at 25 degrees C for 6 months, were compared by DSC, fourier transform infrared microscopic spectroscopy, small angle X-ray diffraction, and scanning electron microscopy. It was confirmed that nearly all the PEO components remain trapped within interlamellar regions of PHB for the PEO/PHB blends before aging. Under this condition, the crystallization of PEO is basically induced by much less active heterogeneities or homogeneous nucleation at high supercoolings. While, after the same PEO/PHB samples were stored at 25 degrees C in vacuum for 6 months, a part of the PEO component was expelled from the interlamellar region of PHB. Under this condition, the expelled PEO forms many separate domains with bigger size and crystallizes at low supercoolings by active heterogeneous nucleation, whereas the crystallization of PEO in the interlamellar region is still mainly induced by less active heterogeneities or homogeneous nucleation at extreme supercoolings. (c) 2005 Wiley Periodicals, Inc.