Exothermic reactions between metals and fluorinated polymers are found in a variety of energetic materials, including reactive binder systems and the Magnesium-Teflon-Viton incendiary composition. This paper describes the reactions between a high molecular weight perfluoropolyether, Fomblin Y 140/13, and magnesium in a variety of morphologies including m-scale powders and nano-scale layered films. Using Temperature Programmed Desorption and Temperature Programmed Reaction we have found that the magnesium-perfluoropolyether interaction is characterized by: (1) competition between Fomblin decomposition and desorption, and (2) magnesium passivation by the formation of magnesium fluoride. Differential Scanning Calorimetry measurements establish a lower-bound estimate of the specific reaction energy of 9.2kJg-1. High molecular weight Fomblin (6500amu) undergoes a competitive reaction/desorption process with desorption occurring at 550K and decomposition at 610K. Decomposition becomes more favorable relative to desorption for higher heating rates and thicker films. Perfluoropolyethers produce several characteristic ions in the 70eV election ionization mass spectra, with the CF3+ ion being the most abundant ion observed during both the molecular desorption and decomposition. Larger fragment ions with masses of 235 and 285amu are observed in relatively high concentrations during desorption and low concentrations during decomposition. The reaction between magnesium and Fomblin begins at 400K, producing CF3+, CO+, and C2F5+ in the electron ionization mass spectrum. We propose that these reactions form a passivating layer of magnesium fluoride that protects the remaining metal as it approaches the magnesium melting point. Most of the reaction takes place at 800K and above when the magnesium fluoride film ruptures.