In the far u.v. photolysis of cis- and trans-2-butene in the gas phase with the xenon and krypton resonance lines (8.4 and 10.0 eV), we show the main decomposition reactions of the excited 2-butene molecules. Main products from these reactions are 1,3-butadiene (0.4), acetylene (0.3), propadiene + propyne (0.2), and ethylene (0.1) at 8.4 eV. The pressure effect on the decomposition reaction of the chemically activated s-butyl radical was also examined. From these studies, it appears that some of the hydrogen atoms are not "thermal", but carry kinetic energy, although this amount is probably not very large. Decomposition reactions of excited 2-butene molecules (8.4 eV) give one hydrogen atom plus a free radical.