Miscibility between poly(ethylene terephthalate-co-ethylene naphthalate) (PTN) and poly(ether imide) (PEI) was investigated by the cloud point method with the aid of time-resolved light scattering analysis on crystallization and liquid-liquid phase separation. The system was found to exhibit upper critical solution temperature (UCST) type phase behaviour. The higher the ethylene naphthalate (N) content in PTN, the lower the UCST and the wider the single-phase region in the phase diagram. From the phase diagrams, the temperature dependence of the binary interaction parameters chi(i/j) between constituent monomeric units (i,j=N, T and EI; T and EI being ethylene terephthalate and ether imide, respectively was estimated. chi(T/N) was a positive but small quantity while chi(N/EI) was negative, suggesting that the miscibility betwen PTN and PEI is mostly caused by an exothermic intermolecular interaction between N and EI units and that the repulsive intramolecular interaction between T and N in the copolymer plays a minor role. Based on the temperature dependence of chi(i/j) a miscibility map (temperature-copolymer composition phase diagram) was established.