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Birefringence measurements of several types have been carried out as a function of temperature on films of polyacrylonitrile and a 7% vinyl acetate copolymer. The stress‐optical coefficient is positive with a value of about +2 Br in the glassy state, and becomes negative as temperature increases through the glass transition region. Pronounced nonlinear stress‐optical behavior is seen in the homopolymer at room temperature and above. Drawn samples have a negative orientation birefringence which changes gradually toward positive values as temperature is increased; the homopolymer and copolymer curves cross zero at about +150° and +195°C, respectively. Temperature cycling of undrawn homopolymer samples under moderate dead load gives birefringence curves which show striking hysteresis effects. The heating curve goes through a minimum at about +158°C, then rises and crosses to positive values at higher temperatures. The cooling curve of samples heated to temperatures above 158°C shows a steep drop to birefringence values of about -0.006 at room temperature (comparable to values obtained by cold‐drawing, even though the sample extension in these experiments is only 10%–15%). Birefringence changes with time in samples under dead load at temperatures below 158°C are in a negative direction, and appear to be associated with mechanical creep. Above 158°C, the time effects are in a positive direction and are associated with the chemical reaction causing discoloration of the polymer. In this region, birefringence changes linearly with time (zero‐order kinetics). Rate of birefringence change is not proportional to applied stress, but its temperature dependence gives an activation energy of 29 kcal/mole which is independent of stress level. At 250°C and above, in samples under dead load, the effects of a second chemical reaction are observed as a rapid increase in birefringence to very high positive- values.