In a process whereby ethylene is copolymerized with another alpha-olefin and a norbornene polyene under pressure with a vanadium catalyst to form amorphous vulcanizable elastomers, it is possible to halogenate the terpolymers in a water slurry by the addition of liquid halogen. The proportion of water to terpolymer crumb in the slurry is critical and must be carefully controlled. The halogenated EPDM terpolymer is cure compatible with unsaturated elastomers such as NR, IR, SBR and BR.