Thionyl fluoride (SOF2) is one of the major sulfur hexafluoride (SF6) decomposition products under electric discharges. The protonation of SOF2 produced by corona discharge of SF6/air mixtures has been studied by the joint application of mass-spectrometric and ab initio theoretical methods. The structurally diagnostic collisionally activated dissociation (CAD) and Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometric results point to the formation of a mixed population of [SOF2]H+ protomers, the HF-SOF+ ion-molecule complex, (H), which reacts with selected nucleophiles as SOF+ donor, and the covalently bounded H-OSF2+ species (1), which behaves as a protonating agent. Computational results at the B3LYP and CCSD(T) levels of theory show that the stability of isomer I exceeds that of 11 by 7.6 and 5.4 kcal mol(-1), respectively, whereas the energy barrier for their interconversion is computed to be as large as 45.2 kcal mol(-1) at B3LYP level. The protonaffinity of thionyl fluoride is estimated from FT-ICR "bracketing" experiments to be 157.7 +/-3 kcal mol(-1), consistent with the value obtained from theoretical calculation at B3LYP and CCSD(T) levels of theory, 153.6 +/- 3 and 155.5 +/- 3 kcal mol(-1), respectively.