The effect of biaxial stretching and annealing on the structure development in ternary blends of PEN/PEI/PEEK on mostly PEN-rich compositions has been investigated. Both PEN and PEEK are melt miscible with PEI. The addition of PEI greatly suppresses the crystallizability of the PEN while enhancing the glass transition temperature. The addition of PEEK to high PEI containing PEN/PEI recovers this crystallizability with the end result being the high Tg materials that can strain harden. The crystallization habit is mostly PEN at PEN-rich corner of the ternary diagram. The increase of PEI concentration beyond 20% eliminates the crystallizability of PEN and addition of as little as 10% PEEK causes these blends to crystallize upon stretching in the rubbery state.