Thermal decomposition of hexamethyldisilane (HMDS) was studied from room temperature to 1310 K using flash pyrolysis vacuum ultraviolet single-photon ionization time-of-flight mass spectrometry (VUV-SPI-TOFMS). Decomposition pathways of HMDS and initial reaction intermediates were also investigated using density functional theory (DFT) at the B3LYP/6-311++G(d,p) level. Unimolecular decomposition reactions of HMDS involving Si-Si and Si-C bond cleavage, as well as decomposition producing Me4Si and :SiMe2 via a three-centered elimination, were determined as the initiation reactions. Me3SiSi(Me)(2)(center dot), Me4Si, Me3Si center dot, and :SiMe2 were major products of the initiation reactions. These initial products were apt to decompose by homolytic reactions. Me2Si=CH2, :SiMe2, and other silene/silylene intermediates preferred decomposing through molecular eliminations. Both homolytic and molecular elimination reactions are important in the pyrolysis of HMDS.