The gas-phase reaction between enolate anions RCOCH(2)(-) and CF3COCl has been studied using FT-ICR. Carbon and oxygen acylation with CF3COCl gives distinct ion products. For the series R = CF3, m-XC(6)H(4) (X = CH3, H, OCH3, F, CF3), H, CH3, (CH3)(3)C, CH2=CH, and F, carbon attack and oxygen attack are both observed with oxygen attack predominating for most of the enolates. This results from the lower intrinsic activation energy for oxygen attack. The relative reactivity of carbon vs oxygen changes significantly with the substituent R in the enolates. The relationship between the relative overall exothermicity of the two channels and the relative reactivity of carbon vs oxygen of enolates is derived from Marcus theory. The relative overall exothermicity of the two channels is correlated with the energy difference, Delta H-ke, between the keto and enol tautomers of the parent ketones.