(2-Mercaptophenyl)phosphonic acid is prepared by hydrolysis of dialkyl (2-mercaptophenyl)phosphonate, which is itself formed either by reaction of dialkyl chlorophosphate with lithium 2-lithio-benzenethiolate, or from the lithiated derivative of the readily prepared O,O-diisopropyl S-phenyl thiophosphate whose phosphoryl group then undergoes an S → C migration.