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Desorption chemical Ionization mass spectrometry (DCI-MS) was Investigated as a technique for obtaining the structural characterization of perfluorlnated polyethers (PFPE). Negatlve-ion DC I mass spectra were dominated by sequences of oligomeric alkoxy fragment Ions, each retaining one of the terminal groups of the polymeric chain. Ion abundances showed Gaussian distributions, which were ascribed to lowenergy deposition during desorption. The most Important process In the gas-phase Ion chemistry of PFPEs resulted In the cleavage of C-0 bonds, associated with charge retention on the oxygen; only low-intensity peaks were due to formation of carbanlons. The high selectivity of this process produced extremely simple mass spectra, even for complex copolymers. The type and molar fraction of monomeric units and terminal groups contained In the analyzed PFPEs were easily Inferred from their DC I mass spectra. Tandem mass spectrometry of oligomeric alkoxy Ions and carbanlons produced by DC I showed that the most favored dissociation was the loss of monomeric units. The mechanism of other fragmentation processes was also Investigated. An Interesting difference was found between metastable and colllsionally activated loss of C"F2nO: the former occurs via fluorine rearrangement, whereas the latter Involves a direct a cleavage at the charge site.