Synthesis of perfluorodialdehydes

A method which comprises reacting 1 equivalent of a compound of the formula ROOC--(CF.sub.2).sub.x --COOR wherein R is alkyl having 1 to 4 carbon atoms and x is a positive integer 2 to 12 with about 1 to 1.5 equivalents of sodium bis-(2-methoxyethoxy)aluminum hydride at a temperature below about -40.degree. C. in a solution of (a) toluene or benzene and (b) an alkyl ether having a freezing point below said reaction temperature and isolating the reaction product, said reaction product being a compound of the formula ##STR11## wherein n is 0 or 1 when said x is less then 4 and said reaction product being a compound of the formula ##STR12## wherein y is 4 to 12 when x is at least 4.