Compared to ethylene and its nonfluorinated derivatives, C2F4 is peculiar in many reactions. It very easily adds to radicals and prefers formation of four-membered rings over Diels-Alder reactions. This has been rationalized by the preference of fluorine for carbon sp(3) hybridization, which is possible on opening of the double bond. Another property, the thermal dissociation of the C=C bond, has been explained by the stabilization of the product (CF2) by back-bonding. Here, it is attempted to correlate such properties with vibrational constants, in particular for C=C stretching and twisting and for carbon pyramidalization. The only force constant found to be lowered compared to ethylene is that for trans pyramidalization (nu(8)), and CC bond softening on nu(8) distortion is indicated by the conspicuously large magnitude of anharmonic constant, x(18). Both observations can be rationalized by a valence-bond model that predicts a trans bent structure on weakening the CC bond. Conclusions are drawn about the dissociation path and peculiarities of the potential. Other anharmonicities, both experimental and calculated and some in (CCF4)-C-12-C-13 and (C2F4)-C-13, are also discussed. In particular some strong Fermi resonances are identified and their effects accounted for.