The alkaline hydrolysis of some five- and six-membered cyclic phosphonium salts has been examined, and the results have been related to current views on the structure and stereochemistry of the phosphorane intermediates of such reactions. Hydrolysis of the iodomethyl salts (IV; R = Me or Ph, X = I, Y = I–) of the 9-substituted 9-phosphafluorenes occurs with ring-expansion to give 9-substituted 9,10-dihydro-9-phosphaphenanthrene 9-oxides (V; R = Me or Ph), whereas hydrolysis of the corresponding salts (XVI; R = Me or Ph) of the 10-substituted phenoxaphosphines occurs with loss of the iodomethyl group and retention of the six-membered ring to give the 10-substituted phenoxaphosphine 10-oxides (XVII; R = Me or Ph). Hydrolysis of the methiodide (XV; R = Ph) of 10-phenylphenoxaphosphine proceeds with fission of the six-membered ring. The conformation of the phenoxaphosphine ring system is discussed. Variable temperature 1H n.m.r. studies indicate that the energy barrier to conformational inversion in this system is very small.