The effect of supercritical CO2 on the morphological structure of compatible crystalline/amorphous poly(ethylene oxide) (PEO)/poly(methyl methacrylate)(PMMA) blends was investigated by means of small angle X-ray scattering (SAXS) with the measurement of absolute scattering intensity. The morphological structure of PEO/PMMA exhibited a considerable change upon CO2 treatment as demonstrated by the drastic increase of scattering intensity, or the enhancement of electron density contrast between the crystalline and amorphous layers in the lamellar stacks, resulting from the swelling of amorphous PEO via the incorporation of CO2 into the interlamellar (IL) regions. Upon CO2 treatment, both the crystal and amorphous layer thickness (l(c) and l(a), respectively) were increased with the extents depending on the blend composition. The increasing of l(a) upon CO2 treatment was decreased with increasing PMMA content, suggesting that PEO was much easier to be swollen by CO2 than PMMA. Compared with the increase of l(a), the increase of l(c) was much more significant and was attributed to the occurrence of melting and recrystallization during CO2 treatment that led to thicker PEO crystals caused by an increased crystal surface free energy. The measured electron density contrast revealed that the distance of segregation in the PEO/PMMA blends involved the extralamellar segregation before CO2 treatments and the swelling of IL region dominated the drastic increase of scattering intensity after CO2 treatments. The finding of extralamellar morphology was consistent with the magnitude of volume fraction of lamellar stacks in the blends. The CO2 treatment could increase the distance of segregation for neat PEO and PEO-rich blends. The lamellar size distribution appeared to be broader and the lamellar stacks more disorganized for the blends after CO2 treatments according to SAXS one-dimensional correlation function profiles. (C) 2003 Elsevier B.V. All rights reserved.