Os(H)(3)ClL2 (L = (PPr3)-Pr-i) reacts at 20 degreesC with vinyl fluoride in the time of mixing to produce OsHFCl(dropCCH(3))L-2 and H-2. In a competitive reaction, the liberated H-2 converts vinyl fluoride to C2H4 and HF in a reaction catalyzed by Os(H)(3)ClL2. A variable-temperature NMR study reveals these reactions proceed through the common intermediate OsHCl(H-2)(H2C=CHF)L-2, via OsClF(=CHMe)L-2 and OsHCl(H-2)(C2H4)L-2, all of which are detected. DFT(B3PW91) calculations of the potential energy and free energy at 298 K of possible intermediates show the importance of entropy to account for their thermodynamic accessibility. Calculations of unimolecular C-F cleavage of coordinated C2H3F confirms the high activation energy of this process. Catalysis by HF is thus suggested to account for the fast observed reactions, and scavenging of HF with NEt3 changes the product to exclusively Os(H)(2)Cl(CCH3)L-2. The analogous reaction of Os(H)(3)ClL2 with H2C=CF2 produces exclusively OsHFCl(=CCH3)L-2 and HF, and the latter is again suggested to catalyze C-F scission via the observed intermediates Os(H)(2)Cl(CF2CH3)L-2 and OsHCl(=CFMe)L-2.