The phase behaviour of melt mixed binary and ternary blends of PEN, PEI and PEEK were investigated using differential scanning calorimetry (d.s.c.) and dynamic mechanical thermal analysis (DMTA) techniques. Both PEN/PEI and PEN/PEEK binary blends exhibit single glass transition temperatures (Tgs) in full composition range and PEN and PEEK were found to be immiscible particularly at mid-concentration range. When PEI is added to the immiscible PEN/PEEK system, first the blends form two separate PEN-rich and PEEK-rich phases below about 40% PEI concentration, and above this concentration, these three homopolymers form a miscible phase in the amorphous state exhibiting a single Tg. Based on the d.s.c. and DMTA results, an approximate ternary phase diagram is drawn. Three component Fox equations were found to describe the Tgs of the ternary blends in the miscible regions quite well.