An investigation has been made of the mercury (3P1) photosensitized polymerization and hydrogenation of butadiene. The main products of the reaction of butadiene with mercury (3P1) atoms are hydrogen, an acetylenic compound in the C4 fraction, dimer, and polymer. In the presence of excess hydrogen, the products are mainly butane, butene, and octanes. The variation of the rate of the polymerization reaction with time, and with the pressure of butadiene, suggests the following mechanism: C4H6+Hg(3P1)→C4H6*+Hg(1S0), followed by C4H6*+C4H6→2C4H6, or C4H6*+C4H6→(C4H6)2 or C4H6*→C4H4+H2. Polymerization arises largely by a free radical mechanism H2+Hg(3P1)→2H+Hg(1S0),H+C4H6→C4H7,C4H7+C4H6→C8H13, etc. The mechanism is shown to be consistent with the value found for the quantum yield.