The effect of transesterification on the phase behaviour in poly(ethylene 2,6-naphthalate) and poly(ethylene terephthalate) blends was investigated by using differential scanning calorimetry. The transesterification between the two polymers was confirmed by LH nuclear magnetic resonance. The blend samples heat treated for up to 11 min at 280 degrees C show two glass transitions, which approach closer with the increase of reaction time. After 13 min, a single glass transition is observed, and the glass transition range becomes narrow with the lapse of reaction time. From the glass transition behaviour, the phase compositions and the phase fractions were determined as functions of reaction time at 280 degrees C. The composition difference between ethylene 2,6-naphthalate (EN)-rich and ethylene terephthalate (ET)-rich phases decreases as the transesterification proceeds. On the other hand, though the weight fractions of the EN-rich phase, the ET-rich one and the interfacial zone tend to change with the lapse of time, the changes are small in magnitude.