A study of the normal modes of vibration and their dispersion in polycytidilic acid based on the Urey-Bradley force field and Raman and Fourier transform infrared spectroscopy is reported. Characteristic features of dispersion curves such as regions of high density of states, repulsion, and character mixing of dispersive modes are discussed. The predictive value of the heat capacity as a function of temperature is reported. V V C 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Poly Phys 44: 504-516, 2006