AbstractThe simultaneous measurement of the birefringence and strain of a sample subjected to sinusoidal strain is proposed as a means for elucidating the molecular mechanism of the response of high polymers to mechanical deformation. A linear phenomenological theory is proposed in which strain‐optical coefficient distribution functions are assigned to the elastic and viscous members of a distribution of Maxwell elements. An apparatus has been constructed to determine the dynamic strain‐optical coefficient at frequencies between 0 and 600 cycles/sec. by means of mechanical excitation of vibration at low frequencies and electromagnetic at high frequencies. The dynamic birefringences of polyethylene has been studied at room temperature over this frequency range.