Thermal degradation of commercial perfluoropolyethers (Fomblin Z and Y, Krytox, and Demnum) and PFPEO (perfluoropoly(ethylene oxide)) in the presence of Lewis acids (metal oxides and halides) was examined. It is shown that the degradation is dominated by the intramolecular disproportionation process of the following scheme (formula omitted) The reaction occurs when the two oxygen atoms flanking a monomer unit of polyether chains interact with a Lewis acid site(s). The differential charges induced at the carbons immediately outside and inside the unit cause a fluorine transfer and effect the disproportionation process. It is shown that the susceptibilities of various sectors of perfluoropolyether chains to the postulated process are determined by the disposition of the oxygens flanking the monomer unit of the sector and by the initial net atomic charges of carbons involved in the fluorine transfer. The disproportionation reaction does not occur at internal sectors of Krytox and PFPEO, for example, on account of the “trans” disposition of the successive ether oxygens; the reaction occurs readily and exclusively at chain ends, however. © 1992, American Chemical Society. All rights reserved.