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A comparison is made of the activities of aerogels, xerogels, and precipitates as catalysts for the dehydration of alcohols, the esterification of acetic acid with ethanol and the dehydration and decarboxylation of aliphatic acids. The aerogels were often, but not always, the most active catalysts. The advantages of the aerogel form are found to be of smaller magnitude than was predicted from earlier work. THIS communication is an extension of the general problem of evaluating aerogels as catalysts, which is being carried out in this laboratory. Kistler, Swann, and Appel (11) showed that thoria aerogel was a more active catalyst than the xerogel (ordinary dried gel) or precipitate for decarboxylating aliphatic acids and esters to ketones. In a study of the vapor-phase oxidation of acetaldehyde to acetic acid over silica and platinized silica, Foster and Keyes (7) showed that the best aerogels and xerogels have about the same activity. These gels appear to be the best vapor-phase