Abstract The major product of the thermal gas-phase addition of CF 3 OF to trichloroethene at 303.8 K was CF 3 O (CHClCCl 2) F. Other products, ie CHClFCCl 2 F and CF 3 O (CHClCCl 2) n⩾ 2− OCF 3, were formed in minor amounts. A reaction mechanism is proposed.