Fluoroantimonic acid is the strongest superacid based on the measured value of its Hammett acidity function ("H"0), which has been determined for different ratios of HF:SbF5. While the "H"0 of pure HF is −15, addition of just 1 mol % of SbF5 lowers it to around −20. However, further addition of SbF5 results in rapidly diminishing returns, with the "H"0 reaching −21 at 10 mol %. The use of an extremely weak base as indicator shows that the lowest attainable "H"0, even with > 50 mol % SbF5, is somewhere between −21 and −23. The following "H"0 values show that fluoroantimonic acid is much stronger than other superacids. Increased acidity is indicated by smaller (in this case, more negative) values of "H"0.