The interconversion of isomeric unsaturated C-4 nitriles (allyl cyanide, crotononitrile and methacrylonitrile) in solution in the presence of butyllithium was investigated by means of FTIR and UV-VIS spectroscopy. The generation and existence of a carbanionic intermediate in the base-induced interconversion of C, nitriles were unequivocally proven and characterized by the shifts in the nu(CN) and nu(C=C) vibration regions of the IR spectra and in the UV-VIS spectra of the reacting nitriles.