Co-pyrolysis of ethynylpentamethyldisilane with 2,3-dimethylbutadiene afforded trimethylsilylacetylene, dimethylsily(trimethylsilyl)acetylene and 1,1,3,4-tetramethyl-1-silacyclopent-3-ene. When an alkyl or trimethylsilyl substituted ethynyl group was pyrolyzed, extrusion of dimethylsilylene was the only decomposition process observed with no isomerization. The decomposition and isomerization pathways were proposed to proceed via a silacyclopropene intermediate. Arrhenius parameters were determined for these processes by using a pulse stirred-flow reactor. The substitution on the silacyclopropene intermediate determined which pathway would be favored. When a good migrating group was on the silicon in the silacyclopropene, a vinylsilylene intermediate was also involved. The decomposition via a silacyclopropene competed with an [alpha]-elimination when both were possible;Pyrolysis of ethynylundecamethylcyclohexasilane afforded the ring-expanded isomer 3-hydridoundecamethylhexasilacyclooctyne and decamethylpentasilacycloheptyne. Pyrolysis of 4,4,5,5,6,6,7,7,8,8-decamethyl-4,5,6,7,8-pentasilacyclooctyne afforded ring-contracted isomer 1,1-penta(dimethylsilylene)propadiene, 4,4,5,5,6,6,7,7-octamethyl-4,5,6,7-tetrasilacycloheptyne, 1, and 1,1-tetra(dimethylsilylene)propadiene, 2. Cycloheptyne 1 was also pyrolyzed and afforded propadiene 2 as the only product. Several mechanisms were proposed for these decompositions and isomerizations. The pathways involving a loss of dimethylsilylene were proposed to proceed through a silacyclopropene intermediate. The isomerizations were proposed to go through a cyclopropene intermediate;Bis (1,3-tetra(dimethylsilylene)) propadiene, 3, was synthesized from cycloheptyne 1 in 20% yield. An x-ray structure of the smallest "betweenallene", 3, was determined. The propadiene carbons were linear and twisted 18 degrees towards planarity. Pyrolysis of propadiene 3 afforded isomer 1,1-3,3-bis(tetra(dimethylsilylene)) propadiene as the only observed product. The proposed mechanism involved cyclopropene intermediates.