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SynopsisThe stress-optical coefficients ( SOCs) of miscible polystyrene (PS) /poly( 2,6-dimethyl-1,4-phenylene oxide) (PPO) blends were measured in the melt state over a wide range of compositions. The flow birefringence measurements were performed with a slit die rheometer using a simplified procedure which eliminates the need to specify the distribution of extinction angles in the flow space. The SOC of the PS/PPO system at -T8 + llO°C follows a linear blending law and full optical compensation (SOC = 0 ) is observed at -61 wt % PS. This value is lower by -10% relative to values obtained by film stretching techniques. Measurements of SOC in the glassy state also yield linear dependence of SOC on composition but no compensation is obtained in this case because of the sign reversal in the SOC of PS in the glass transition range.