The photolysis of 1,3-butadiene in the vapor phase was studied in the presence of inert gases such as krypton, diethyl ether, and cyclohexane. In every instance, an increase in the pressure of the added gas led to a decrease in all of the volatile products. The behavior of 1,2-butadiene and 2-butyne were exceptional in that they showed a slight increase in yield when a small quantity of an inert gas was added. But at higher pressures, even in these cases, the yields decreased. A consideration of the radiative lifetime of the first-singlet excited state of 1,3-butadiene suggests that the observed pressure dependence cannot be attributed to a quenching reaction from that state. It seems likely that all of the volatile products in the vapor phase photolysis arise from a vibrationally excited ground state molecule of 1,3-butadiene that is formed by the internal conversion of the electronically excited singlet state. Photolysis of 1,3-butadiene in solution, which is equivalent to photolysis at very high pressures, yielded none of the volatile products that were observed in the vapor phase but only cyclobutene, bicyclo [1.1.0] butane, dimers, and a polymer. The cyclobutene and bicyclobutane are believed to be products of isomerization from the electronically excited state of 1,3-butadiene. The formation of a polymer in the gas-phase photolysis was found to have an induction period and to depend on the square of the number of photons that were absorbed. In all probability, the polymer is formed in the photolysis of a volatile product (s) of the primary photodecomposition.