The phase behaviour and crystallization of poly(ethylene-2,6-naphthalenedicarboxylate) (PEN)/poly(ether imide) (PEI) blends were investigated. PEN and PEI were miscible in the melt over the entire composition range. A PEI-rich phase containing ca. 80 wt.% of PEI was identified after the crystallization of PEN below 240 degrees C. This observed composition deviated from the composition calculated by assuming a simple liquid-solid phase separation. Morphological observation by optical microscopy revealed simultaneous occurrence of spinodal decomposition and crystallization, where modulated structure was locked in by the crystallization of PEN. A UCST phase diagram with the binodal line located below the equilibrium melting point was identified. Investigation on the multiple melting behaviour showed that recrystallization of PEN following the initial melting was hindered due to the remixing between PEN and PEI. Despite the occurrence of spinodal decomposition, the crystallization rate of PEN still dropped with increasing initial PEI composition.