Oxophosphoranesulphenyl chlorides react readily with esters of tercovalent phosphorus acids giving, in addition to thiopyrophosphate systems P(O)–S–P(O), acid chlorides and O-esters P(S)OR. The symmetrical thiopyrophosphates are converted into their unsymmetrical isomers, P(O)–O–P(S), during the isolation procedure. The reaction is seen to involve nucleophilic attack by the phosphorus atom on the sulphur atom of the oxophosphoranesulphenyl chloride, and formation of a quasiphosphonium complex which decomposes by two parallel paths.