The kinetic parameters were determined for C-trifluoromethylation of aniline with S-(trifluoromethyl)dibenzothiophenium triflate (1), its 3,7-dinitro derivative (2) and S-(trifluoromethyl)diphenylsulfonium triflate (3) in DMF-d 7 . The higher reactivity of heterocyclic 1 compared with non-heterocyclic 3 could be explained on the basis of its greatly enhanced activation entropy (ΔS ≠ : - 11.2 cal mol - 1 K - 1 for 1; -47.1 for 3), but not its enhanced activation enthalpy (ΔH ≠ : 21.2 kcal mol - 1 for 1; 12.1 for 3). The aromatic delocalization of the heterocyclic ring may thus be only slightly disturbed by the S-trifluoromethyl substituent. The high reactivity of 2 was attributed to the great electron deficiency caused by two nitro groups in addition to the heterocyclic salt system (ΔH ≠ 17.0 kcal mol - 1 , ΔS ≠ -9.1 cal mol - 1 K - 1 for 2). The reaction mechanism is discussed; the conventional S N 2 attack mechanism was ruled out and a mechanism for a side-on attack to the CF 3 -S bond may possibly be applicable.