Stress optical behavior as influenced by blend composition has been investigated by on-line stress monitoring during melt spinning of a series of melt miscible PEEK/PEI blends. At low stresses/draw downratios, the fibers are found to be amorphous. In this range, the linear stress optical rule is obeyed and stress optical constant was found to be composition dependent. Beyond a critical drawdown, the PEEK chains first form very highly oriented ordered domains that exhibit significant translational disorder along the fiber axis. In the processing range observed, this structure persists even at high spinline deformation/stresses. While the stress optical behavior remains linear for those compositions where no stress crystallization was detected, negative deviation from initial linearity was observed when these ordered PEEK regions began to be observed indicating that birefringence saturation is approached while stresses are increased.