A study of the stereochemistry of substitution reactions at the phosphorus atom of 2,2,3,4,4-pentamethylphosphetans has been made. It is proposed that all such displacements proceed via trigonal bipyramidal intermediates in which the ring spans an apical-equatorial position. Walden cycles in which substitution at the phosphoryl centre of phosphetans proceeds with retention of configuration are described and a mechanism involving pseudorotation of the intermediates is proposed. Cis and trans isomers of 2,2,3,4,4-pentamethylphosphetan 1-oxides can be distinguished by the paramagnetic shifts of the respective 3-protons in their n.m.r. spectra in the presence of tris (dipivalomethanato) europium (III). Following a discussion of the mechanism of the alkaline hydrolysis of phosphonium salts, the hydrolysis of three salts in D2O-H2O (1:1) is described. The kinetic isotope effects observed show that the carbanions involved in these hydrolyses are not free in the rate-determining transition state. The alkaline hydrolysis of a 3-pbospholenium salt is described in which the normal rule, that hydrocarbon will be formed from the group most stable as the anion, does not apply. 2,2,3,4,4-Pentamethylphosphetanium salts undergo alkaline hydrolysis with complete retention or with partial inversion of configuration at phosphorus. These differences are discussed in terms of the energy barriers to pseudo-rotation of the intermediates involved. Other substitution reactions at the phosphorus of phosphetans bring about either ring expansion or ring opening. All the reactions are governed by the preference of the four-membered ring for the apical-equatorial position in the trigonal bipyramidal intermediates. Attempts to generate the ylide from 1,2,2,3,4,4-hexamethyl-1-phenylphosphetanium iodide lead to ring opening via pentacovalent intermediates as also do the action of cyanide ion on this salt. 2,2,3,4,4-Pentamethyl-1-phenylphosphetan 1-sulphide with two molar proportions of phenyl-lithium gave (1,1,2,3-tetramethyl-3-phenylbutyl)diphenylphosphine. 2,2,3,4,4-Pentamethyl-1-phenylphosphetan with chlorine or bromine gave (1,1,2,3-tetramethyl-but-3-enyl) halogenophosphines which cyclised to phospholens on heating, and to phospholens and phosphetan 1-oxides on treatment with aluminium chloride. A study was made of the conversion of thiophosphoryl compounds into phosphoryl compounds.