The rate of solvolysis of ethyl trifluoroacetate has been investigated in a series of H2O/D2O mixtures at 9.97 °C. The secondary isotope effects related to the hydrolysis of the isotopic pairs, methyl/methyl-d3, ethyl/ethyl-d5,i-propyl/i-propyl-β-d6andt-butyl/t-butyl-d9trifluoroacetates have been established as well as the temperature dependence for the hydrolysis oft-butyl trifluoroacetate. The significance of all these measurements with respect to the mechanism of these reactions is discussed.