The thermal decomposition of isoprene up to 1400 K was performed by flash pyrolysis with an similar to 100 mu s time scale. This pyrolysis was followed by supersonic expansion to isolate the reactive intermediates and initial products, and detection was accomplished by vacuum ultraviolet single photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS) at lambda = 118.2 nm. Products CH3, C2H4, C3H3, C3H4, C4H4, C4H5, C5H6, C5H7, and C6H6 were directly observed and provide mechanistic insights to the isoprene pyrolysis. At temperatures >=similar to 1200 K, the molecular elimination of ethene to form C3H4 and sigma bond homolysis producing C4H5 and CH3 radicals are competitive reaction pathways. The molecular elimination of acetylene to form C3H6 was minimal and direct C2-C3 sigma bond homolysis was not observed. The C3H3 radicals are also observed, as a result of hydrogen loss of C3H4 by pyrolysis or hydrogen abstraction by the CH3 radical from C3H4. Above similar to 1250 K, production of C6H6 was observed and identified as the combination product of the C3H3 radicals.