The kinetics of the thermal reaction of FS(O2)OO(O2)SF with SO2 have been theoretically studied. Experimental investigations performed at 293–323 K indicate that the FSO3 radical, in equilibrium with the peroxide FS(O2)OO(O2)SF ¢ 2 FSO3 (1, -1), initially attacks the SO2 forming the FS(O2)OSO2 radical which afterwards may dissociate back, FSO3 + SO2 ¢FS(O2)OSO2 (2, -2), or recombinewith FSO3 generating the final product, FSO3 + FS(O2)OSO2? (FS(O2)O)2SO2 (3). Several DFT formulations and composite ab initio models were employed to characterize FS(O2)OSO2 molecular properties and to determine relevant potential energy surfaces features of reactions (2), (-2) and (3). Transition state theory calculations lead to the high pressure rate coefficients k1;2 ¼ 9:1 10 14 expð 5:2 kcal mol 1 = RTÞ cm3 molecule 1 s 1 and k1; 2 ¼ 4:9 1015 expð 13:9 kcal mol 1 =RTÞ s 1 while statistical adiabatic channel model (SACM/CT) calculations predict for the barrierless reaction (3) the expression k1;3 ¼ 2:9 10 11ðT=300Þ0:4 cm3 molecule 1 s 1. The experimental phenomenological rate coefficients are very well reproduced by these rate coefficients.Facultad de Ciencias Exactas