A low frequency Raman band assignable to the longitudinal acoustical mode (LAM) in an alternating copolymer of ethylene and chlorotrifluoroethylene (E/CTFE) has been observed. The anisotropic scattering properties of an oriented filament have been determined by polarized Raman measurements and the results confirm this assignment. The frequency-chain length variation characteristic of the LAM has also been observed and studied through carefully-controlled annealing experiments. The dynamics of the 150°C crystal-crystal phase transition was investigated using the bandwidth and frequency position of the LAM as a molecular probe. It was found that its shift to lower frequency upon crossing the phase boundary reflected a decrease in the c-axis Young's modulus consistent with a deviation of the backbone conformation away from a planar structure. © 1981.