The photolysis of diazomethane in the presence of trans butene-2 and cis butene-2 has been investigated. The results are explained on the basis that the methylene initially produced may be vibrationally excited, and may also contain considerable excess kinetic energy. The methylene reacts with the butenes before this excess energy is lost. The trans and cis 1.2-dimethylcyclopropanes produced by the addition of the methylene to the double bond in trans and cis butene-2, respectively, undergo both geometrical and structural isomerization unless collisionally stabilized. The structural isomerization, which is considerably slower than the geometrical isomerization, results in the formation of cis and trans pentene-2, 2-methylbutene-2 and 2-methylbutene-1.