The reaction mechanism of the sulfur oxygen and/or carbon oxygen bond breaking in sulfonates during nucleophilic substitution has been studied using DFT (B3LYP) method at 6-31G level. Through the analysis of the kinetics and thermodynamics, we conclude that halide ion (F-) easily attacks the sulfur atom of perfluoroalkanesulfonic ester, causing the S-O bond cleavage to give CF3SO2F and -OCF2CF3. In contrast to the S-O bond cleavage it has been found that the C-O bond scission in perfluoroalkanesulfonic ester is difficult to take place. This conclusion is in good agreement with experiment. (C) 2002 Elsevier Science B.V. All rights reserved.