A small-angle X-ray scattering (SAXS) study was carried out to investigate the morphology of blends of poly(ε-caprolactone) (PCL) and polystyrene oligomer (PSO) when the former component was crystallized. This PCL-PSO system shows an upper critical solution temperature type of phase diagram. For blends with ф PCL < 0.8 ( ф PCL is the weight fraction of PCL), the scattering observed is the superposition of the intensities arising from the crystalline region (made up by alternate stacking of lamellae and thin amorphous layers) and from the inhomogeneity in the system (due to the crystalline regions and the amorphous domains). For blends with ф PCL > 0.8 , only scattering from the crystalline region was observed. From the SAXS curve, the lamella thickness l c, the amorphous layer thickness l a and the linear crystallinity χ c of the crystalline region were obtained on the basis of the Hosemann-Tsvankin model. The value of l c was 5.5 nm independent of composition, while l a and χ c were 8.1 nm and 0.4 for blends with ф PCL <0.8 but varied linearly with composition for blends with ф PCL > 0.8 . These parameters were also studied by the correlation function from Vonk's model.