Perfluorinated polyethers are a class of substances which are extremely inert and have both extraordinary high temperature stability and low temperature properties. The synthesis of perfluorinated polyformaldehydes with the highest oxygen content was designed to give the maximum liquid range and low temperature properties. Novel low molecular weight perfluorinated polyformaldehydes with stable and unreactive perfluoro-n-butyl end groups were prepared by liquid-phase direct fluorination. The boiling point of these compounds increases by approximately 20 degrees C with the addition of each difluoromethylene oxide unit. This trend does not continue for longer chain lengths (n > 4) where the increase in boiling point per CF2O unit diminishes. The average increase of melting temperature is similar to 1-2 degrees C as the perfluorinated polyformaldehyde chain increases one difluoromethylene oxide unit. The new perfluoropolyether fluids produced have melting points ranging from -145 to -152 degrees C.