The phase behaviour of amorphous and semicrystalline poly(butylene terephthalate) (PBT)/poly(ether imide) (PEI) blends has been investigated. PET and PEI were miscible in the melt over the entire composition range. Upon the crystallization of PET at temperatures lower than 210 degrees C, a strong segregation of PEI was observed. The morphological observation by optical microscopy indicated that PET crystallization was coupled with a liquid-liquid phase separation, and the modulated morphology created in the early stage of spinodal decomposition was preserved due to the rapid crystallization of PET. The phase diagram for PBT/PEI blends was also determined. This binary pair exhibited a UCST phase diagram with the binodal line located below the equilibrium melting point. Investigation on the multiple melting behaviour showed that recrystallization of PBT after the initial melting was hindered by the presence of PEI. The results presented here have indicated that PBT/PEI exhibited quite similar phase behaviour to poly(ethylene terephthalate) (PET)/PEI blends.