Isobutene reacts readily with excess trifluoroacetic acid in ethylene dichloride solution at ordinary temperatures to givet-butyl trifluoroacetate. The rate of the reaction is given, within the range of the experiments, by the expression d[ester]/dt=k[acid]2[olefin], and the apparent activation energy is about 6 kcal/mole. The rate of addition is markedly dependent on the strength of the reacting acid and is drastically reduced in the presence of mildly basic materials, such as dioxane. The boron fluoride catalyzed addition of acetic acid to 2-butene can be considered to follow a similar rate law, i.e. d[ester]/dt=k[acid·BF3]2[olefin], but only if some assumptions are made about the position of the equilibriumsince only the 1:1 complex is reactive.