An improved process for the polymerization of epoxides with an organoaluminum catalyst is described. Improvements in conversion, yield and reaction rate are realized by contacting the epoxide at a temperature below 0.degree. C. with at least a catalytic amount of the organoaluminum compound in the presence of at least 0.1 mole, per mole of aluminum, of a perhalogenated aliphatic monoketone which is preferably hexafluoroacetone.