Fluoroantimonic acid is formed by combining hydrogen fluoride and antimony pentafluoride:
:SbF + 2 HF   + HF
The speciation (i.e., the inventory of components) of "fluoroantimonic acid" is complex.  Spectroscopic measurements show that fluoroantimonic acid consists of a mixture of HF-solvated protons, [ (such as ), and SbF-adducts of fluoride, [(SbF)F] (such as ). Thus, the formula "" is a convenient but oversimplified approximation of the true composition.  Nevertheless, the extreme acidity of this mixture is evident from the exceptionally poor proton-accepting ability of the species present in solution. Hydrogen fluoride, a weak acid in aqueous solution that is normally not thought to have any appreciable Brønsted basicity at all, is in fact the strongest Brønsted base in the mixture, protonating to HF in the same way water protonates to HO in aqueous acid.  As a result, the acid is often said to contain "naked protons", though the "free" protons are, in fact, always bonded to hydrogen fluoride molecules. It is the fluoronium ion that accounts for fluoroantimonic acid's extreme acidity.  The protons easily migrate through the solution, moving from HF to HF, when present, by the Grotthuss mechanism.
Two related products have been crystallized from HF-SbF mixtures, and both have been analyzed by single crystal X-ray crystallography. These salts have the formulas  and . In both salts, the anion is . As mentioned above,  is weakly basic; the larger anion  is expected to be a still weaker base.