Nafion-H, a perfluorinated sulfonic acid resin, catalyzed the intramolecular Friedel-Crafts acylation of diarylalkane-2-carboxylic acids and arylalkanoic acids in refluxing p-xylene to provide cyclic ketones. The reactions were clean, and the water that was formed as a byproduct did not deactivate the catalyst. It was also found that the intramolecular acylation of the corresponding acid chlorides occurred under much milder reaction conditions. The mechanism of the reaction is discussed.