Ab initio electronic structure calculations provide potential energy surfaces for the decomposition of CF3CFHO and its competing reaction with O-2. Additional calculations with the same methods study the analogous reactions of the unfluorinated analogue CH3CH2O. We use transition state theory, incorporating data from the ab initio computations, to model rate constants for all of these reactions. Comparisons are made between fluorinated and unfluorinated systems as well as to experiment.