triplet-4-Methylene-2-pentene-1,5-diyl (T-4t) has been generated upon irradiation of 2-vinylmethylenecyclopropane (6) in a bromine-doped xenon matrix and characterized by means of IR spectroscopy for the first time. Conclusions for the thermal isomerization of 6 to 3-methylenecyclopentene (7) are discussed. All experimental results indicate that this isomerization probably proceeds via the diradical S-4 as an intermediate.