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Results obtained with semiempirical MO methods support a mechanism for the pyrolysis of vinylphosphirane yielding phosphapropyne in which extrusion of ethylene leads to a vinylphosphinidene intermediate. A pathway from vinylphosphinidene to phosphapropyne involving 2H-phosphirene and 2-phosphapropenylidene intermediates is predicted from high level ab initio results to have a higher energy barrier than that for direct rearrangement. A higher energy pathway from vinylphosphirane is the migration of a hydrogen atom forming methyl(1-phosphiranyl)methylene, followed by loss of ethylene to yield the phosphaalkyne product. However, since fragmentation of 1-phosphiranylmethylene, once formed, has a low predicted barrier, its generation emerged as a new route to phosphaalkynes. (Trimethylsilyl)(1-phosphiranyl)diazomethane was synthesized, and its pyrolysis yields Me3SiC⋮P.