A DECREASE IN PERMEABILITY AND AN INCREASE IN CHEMICAL RESISTANCE OF CARBOXYNITROSO RUBBERS CAN BE EFFECTED BY THE ADDITION OF SMALL AMOUNTS OF POLYBUTADIENE RESINS AND POWDERED TETRAFLUOROETHYLENE POLYMER FILLER. LIQUID FUNCTIONALLY TERMINATED POLYBUTADIENE AND POWDERED POLYTETRAFLUOROETHYLENE ARE MIXED INTO A TERPOLYMER CONTAINING TRIFLUORONITROSOMETHANE, TETRAFLUOROETHYLENE, AND A SMALL AMOUNT OF NITROSOPERFLUOROBUTYRIC ACID. THE MIXTURE IS CURED IN THE PRESENCE OF A PEROXIDE FREE RADICAL INITIATOR.