Fluoroantimonic acid is a mixture of hydrogen fluoride and antimony penta­fluoride, containing various cations and anions (the simplest being H2F+ and Sb F−6).

[1] Like its precursor hydrogen fluoride, it attacks glass, but can be stored in containers lined with PTFE (Teflon) or PFA.

Spectroscopic measurements show that fluoroantimonic acid consists of a mixture of HF-solvated protons, [(HF)nH]+ (such as H3F+2), and SbF5-adducts of fluoride, [(SbF5)nF]– (such as Sb4F−21).

The pKa on the other hand, measures the equilibrium of proton dissociation of a discrete chemical species when dissolved in a particular solvent.

[citation needed] The gas-phase acidity (GPA) of individual species present in the mixture have been calculated using density functional theory methods.

[2] (Solution-phase pKas of these species can, in principle, be estimated by taking into account solvation energies, but do not appear to be reported in the literature as of 2019.)

[citation needed] Fluoroantimonic acid solutions decompose when heated, generating free hydrogen fluoride gas and liquid antimony pentafluoride at a temperature of 40 °C.

In 1967, Bickel and Hogeveen showed that 2HF·SbF5 reacts with isobutane and neopentane to form carbenium ions:[14][15] It is also used in the synthesis of tetraxenonogold complexes.

[10] Safety gear must be worn at all times when handling or going anywhere near this corrosive substance, as fluoroantimonic acid can protonate every compound in the human body.