. Can. J. Chem. 54,440 (1976). The HR acidity function has been determined for the trifluoroacetic acid -water system, and extended beyond 100% acid by the addition of trifluoroacetic anhydride, to 99.5 wt.7, anhydride. The existing Ho acidity function is corrected and extended to 1007, trifluoroacetic acid, confirming Eaborn's observation that Ho attains a minimum value at approximately 977, acid.Comparison of the acidity functions shows that although trifluoroacetic acid is only weakly protonating, it is a strongly protonating-dehydrating medium. Changes in the uv-visible spectrum of potassium permanganate produced by changing acid concentration are used to calculate the degree of further ionization of the permanganate ion; the correlation of this with the HR acidity function further suggests the presence of the permanganyl cation, MnO3+, or its trifluoroacetate complex or ester, MnO3.02CCF3, in these solutions. A cryoscopic investigation into the mode of ionization of permanganate in trifluoroacetic acid reveals that extensive ion-pair formation occurs in this solvent. Although this effect prevents an unequivocal determination of the number of solute particles being made the results are consistent with the presence of a permanganyl species in solution.