The thermal decomposition of 1,3-butadiene, 1,3-butadiene-1,1,4,4-d(4), 1,2-butadiene, and 2-butyne at temperatures up to 1520 K was carried out by flash pyrolysis on a similar to 20 mu s time scale. The reaction products were isolated by supersonic expansion and detected by single-photon (lambda = 118 nm) vacuum-ultraviolet time-of-flight mass spectrometry (VUV-TOFMS). Direct detection of CH3 and C3H3, as well as C3H4, C4H4, and C4H5 products, provides insight into the initial steps involved in the complex pyrolysis of these C4H6 species below T = 1500 K. The similar pyrolysis product distributions for the C4H6 isomers on such a short time scale support the previously proposed mechanism of facile isomerization of these species. Isomerization of 1,3-butadiene to 1,2-butadiene and subsequent C-C bond fission of 1,2-butadiene to produce CH3 and C3H3 (propargyl) are most likely the primary initial radical production channel in the 1,3-butadiene pyrolysis.