The microstructure of cold-crystallized poly(ether- ether-ketone)/poly(ether-imide) (PEEK/ PEI) blends has been investigated by a combination of wide- and small- angle X-ray scattering (SAXS), dielectric spectroscopy, and transmission electron microscopy. Both SAXS and dielectric spectroscopy indicate that most PEI is excluded fi om interlamellar regions and segregates essentially into interfibrillar regions upon crystallization. Interlamellar inclusion was found to be limited to low PEI levels. Transmission electron microscopy (TEM) observations on stained thin sections from bulk samples confirm these conclusions. The restricted character of the interlamellar inclusion of PEI is probably due to a more ordered structure of noncrystalline regions near PEEK crystal surfaces. In addition, our results on PEEK/PEI blends corroborate the model of cold-crystallized pure PEEK which consists of thin lamellar crystals separated by thicker noncrystalline interlayers that are grouped into stacks of limited coherence and uniformly fill the entire sample volume without any separate amorphous pockets or gaps.