The Au(III) complex Au(OAcF)(2)(tpy) (1, OAcF = OCOCF3; tpy = 2-p-tolylpyridine) undergoes reversible dissociation of the OAcF ligand trans to C, as seen by F-19 NMR. In dichloromethane or trifluoroacetic acid (TFA), the reaction between 1 and ethylene produces Au(OAcF)(CH2CH2OAcF)(tpy) (2). The reaction is a formal insertion of the olefin into the Au-O bond trans to N. In TFA this reaction occurs M less than 5 min at ambient temperature, while 1 day is required in dichloromethane. In trifluoroethanol (TFE), Au(OAcF)(CH2CH2OCH2CF3)(tpy) (3) is formed the major product. Both 2 and 3 have been characterized by X-ray crystallography. In TFA/TFE mixtures, 2 and 3 are equilibrium with a slight thermodynamic preference for 2 over 3. Exposure of 2 to ethylene-d(4) in TFA caused exchange of ethylene-d4 for ethylene at room temperature. The reaction of 1 with cis-1,2-dideuterioethylene furnished Au(OAcF)(threo-CHDCHDOAcF)(tpy), consistent with an overall anti addition to ethylene. DFT(PBEO-D3) calculations indicate that the first step of the formal insertion is an associative substitution of the OAcF trans to N by ethylene. Addition of free (-OAcF) to coordinated ethylene furnishes 2. While substitution of OAcF by ethylene trans to C has a lower barrier, the kinetic and thermodynamic preference of 2 over the isomer with CH2CH2OAcF trans to C accounts for the selective formation of 2. The DFT calculations suggest that the higher reaction rates observed in TFA and TFE compared with CH2Cl2 arise from stabilization of the (-OAcF) anion lost during the first reaction step.