Measurements are reported of the catalytic effect of aqueous hydrogen fluoride and fluoride buffer solutions in the following eight reactions: the hydrolysis of diethyl acetal, the depolymerization of paraldehyde, the decomposition of nitramide, the bromination of ethyl acetoacetate and of ethyl cyclopentanone-2-carboxylate, the iodination of acetonylacetone and of acetone, and the dehydration of acetaldehyde hydrate. In the first two, which are specifically catalyzed by hydrogen ions, the catalytic effect of the fluoride systems corresponds to the calculated hydrogen-ion concentrations, and it is suggested that under some conditions fluoride buffer solutions may be particularly useful for controlling pH. In the remaining six reactions there is also general acid-base catalysis by hydrogen fluoride molecules, or by fluoride ions, or by both. The catalytic effects of these two species are close to those predicted by relations previously established for carboxylic acids and their anions. The bifluoride ion has approximately the same effect as hydrogen fluoride in acid catalysis but has no detectable catalytic effect as a base. These findings are discussed in terms of the structures and thermodynamic properties of the species concerned.