Polymerization of tetrafluoroethylene in monochlorodifluoromethane was carried out at low temperatures with -prays from a60Co source. An activation energy of 4.3 kcal/mole was obtained for the in-source polymerization, and this is higher than that of bulk polymerization, 2.7 kcal/mole. It was found that a remarkable postpolymerization takes place even if the reaction system is in liquid state. A kinetic treatment for the postpolymerization is described.