Three stable alkylidenephosphiranes have been synthesized from the addition of the terminal phosphinidene complex Ph-P-W(CO)5 to allene, 1,1-dimethylallene, and tetramethylallene. Isopropylidenephosphirane 16b was characterized by a single-crystal X-ray structure determination. Demetalation of its W(CO)5 group provides the uncamplexed compound. The addition reaction with tetramethylallene also yields vinylphosphirane epimers, which rearrange to phospholene 20. Ab initio MP2/6-31G* structures and energies are presented for the parent uncomplexed methylenephosphirane and its dimethyl derivatives.