Abstract The acidity on the “proton affinity” scale was determined by IR spectroscopy of the pyridinium salts for nineteen heteropoly acids of nine structural types (including two with the previously unknown structure) and one isopoly acid. All heteropoly acids exhibited a high acidity at the level of CF 3 SO 3 H and HClO 4. H 3 PW 12 O 40 was the strongest acid.
