The imidazolinium and benzimidazolium bromide salts with pentafluor substituents on N atom were synthesized. The structures of imidazolinium and benzimidazolium bromide salts obtained were conformed by H-1 and C-13 NMR, F-19 NMR and elemental analysis. It was found that pyrolytic decomposition occurs with melting in salts. The imidazolinium and benzimidazolium bromide salts were studied by TG-DTG and DTA from ambient temperature to 1000 degrees C in nitrogen atmosphere. The decomposition occurred mainly in one stage and the values of activation energy E, frequency factor A, reaction order n, enthalpy change Delta H-#, entropy change Delta S-# and Gibbs free energy Delta G(#), of the thermal decomposition were calculated by means of Coats-Redfern (CR), MacCallum-Tanner (MC) and van Krevelen (vK) methods. The activation energy value obtained by CR and MC methods were in good agreement with each other while those obtained by vK were found to be 10-12 kJ mol(-1) larger.