The thermal decomposition mechanism of thiophene has been investigated both experimentally and theoretically. Thermal decomposition experiments were done using a 1 mm X 3 cm pulsed silicon carbide microtubular reactor, C4H4S + Delta -> Products. Unlike previous studies these experiments were able to identify the initial thiophene decomposition products. Thiophene was entrained in either Ar, Ne, or He carrier gas, passed through a heated (300-1700 K) SiC microtubular reactor (roughly <= 100 mu s residence time), and exited into a vacuum chamber. The resultant molecular beam was probed by photoionization mass spectroscopy and IR spectroscopy. The pyrolysis mechanisms of thiophene were also investigated with the CBS-QB3 method using UB3LYP/6311++G(2d,p), optimized geometries. In particular, these electronic structure methods were used to explore pathways for the formation of elemental sulfur as well as for the formation, of H2S and 1,3-butadiyne. Thiophene was found to undergo unimolecular decomposition by five pathways: C4H4S -> (1) S=C=CH2 + HCCH, (2) CS + HCCCH3, (3) HCS + HCCCH2, (4) H2S + HCC-CCH, and (5) S + HCC-CH=CH2. The experimental and theoretical findings are in excellent agreement.