ABSTRACT Apparatus is described to prepare hydrofluoric acid of high purity. The conductances of potassium fluoride and of the methyl substituted benzenes, including benzene, have been measured in this acid at several concentrations at 20.0. The conductances of these same solutions were also measured in the presence of boron trifluoride up to a pressure of 100 lb. per sq. in. It has been shown that the principal equilibrium existing in solutions of methylbenzenes in hydrofluoric acid is Ar + HF ArH+ + F-. The stoichiometric equilibrium constant for this reaction calculated from the conductance data ranged from 10-8 for benzene to 10-2 for hexamethylbenzene. This order of the basic strengths of the methylbenzenes relative to hydrofluoric acid is substantially in agreement with the results of previous workers arrived at by other techniques. Our results indicate that the boron trifluoride reacts only with the fluoride ion. Isomerization of the tetramethylbenzenes, but not of the xylenes or trimethylbenzenes in anhydrous hydrofluoric acid is reported, and the role of hydrofluoric acid-boron trifluoride mixtures as catalysts in organic reactions is discussed briefly. Our results indicate that the ArH+ cation can be quite stable and is an important intermediate in these systems.