We report a study of the phonon density of states in tetrabromoethylene. C2Br4, a substituted ethylene compound and analogous to tetracyanoethylene, by coherent inelastic neutron scattering measurements and theoretical calculations using extended external mode formalism. It is shown that the density of states that occur below 8 meV correspond to external modes, All the observed internal modes have also been assigned. (C) 2002 Published by Elsevier Science B.V.