Abstract The reaction of o-and p-brominated diphenyl sulfides with phosphorus trichloride in the presence of aluminium chloride has been investigated. After hydrolysis, the reaction yields p-substituted arylthiobenzenephosphonic acids and bromide ion. Similar results are obtained with brominated diphenyl ethers. A mechanism for this “debrominative phosphorylation”, involving a stabilized σ-complex 8, is suggested on the strength of ...