Perfluorobenzoyl peroxide decomposes in benzene and in nitrobenzene at 80° to give mainly pentafluorobiaryls and pentafluorobenzoic acid. In hexafluorobenzene the main product is a high-boiling residue, and in chloro- and bromo-benzene pentafluorobenzoic acid and phenyl pentafluorobenzoate are formed. The latter product must arise by benzoyloxylation of the halogenobenzenes at the 1-position. The results are discussed.