AbstractThe principles underlying the dynamic birefringence method are outlined, giving particular emphasis to the use of this technique in investigating the molecular mechanism of relaxation in amorphous polymers. Data are presented for amorphous polyacetaldehyde in the frequency range 1‐20 cycles/sec. and temperature range 0 to −25°C. The results are discussed and compared briefly with dielectric data for poly‐acetaldehyde and also with earlier dynamic birefringence data obtained for polymethyl‐