Alkaline hydrolysis of 1-iodomethyl-1,2,5-triphenylphospholium iodide gives 3,4,5-triphenyl-4-phosphabicyclo[3.1.0]hex-2-ene-4-oxide in very high yield. Three possible mechanisms are discussed and isolation of a key intermediate allows one of these mechanisms to be discarded. Alkaline hydrolysis of 1-allyl-1,2,5-triphenylphospholium bromide in methanol gives 1,2,5-triphenylphosphole and small quantities of a compound C26H27O2P which is an adduct of the original phospholium ion, hydroxide ion, and methanol.