Lattice dynamics calculations of the lattice modes (phonons) and the internal modes (vibrons) and Raman spectra at liquid helium temperatures are presented for the beta -, alpha - and gamma -phase of 1,4-dichlorobenzene (DCB). It follows from the calculations and it is confirmed by the Raman spectra that these three phases are characteristically different with respect to the following properties: vibron band dispersion, Davydov splitting, ³⁵Cl/³⁷Cl isotopic effects on the band structure and vibron-phonon mixing. Only the low temperature gamma -phase shows a nice one-dimensional band structure for some vibrations. The vibron band dispersion in all phases of DCB is caused by the repulsive exponential terms in the intermolecular potential. This is in contrast to 1,2,4,5-tetrachlorobenzene, where the electrostatic interactions are responsible for the dispersion. Some of the ungerade vibron modes become visible in the Raman spectra of DCB due to the (random) occurrence of molecules with different distributions of ³⁵Cl/³⁷Cl isotopes. A few of the Raman lines observed could not be uniquely assigned; they are probably caused by Fermi resonances.