To improve the physical properties of polyethylene, the method of fluorinating polyethylene powder was investigated. Batch reaction vessels were used. To prevent a fiery, intense reaction, the following are effective: use of fluorine at low pressures; use of helium or nitrogen as diluent for fluorine; choice of reaction-vessel shape to minimize fluorine velocity on a powder sample when fluorine is supplied; keeping a polyethylene sample in vacuum for a sufficient time before fluorination starts; and raising the reaction temperature to about 90° C. The weight increase of polyethylene during fluorination is proportional to the 0.28th power of time. © 1969, American Chemical Society. All rights reserved.