Titania-functionalized monolith catalysts operated at short contact times are highly selective for the formation of ketones from carboxylic acids. The selectivity of ketonization of acetic acid to form acetone over such catalysts varies little as the conversion is varied from 3 to 100%. These results demonstrate that ketonization is not a sequential reaction proceeding via a gaseous ketene product. The selectivity for ketene versus ketone formation is determined by the intrinsic chemistry of the catalyst, and cannot be manipulated by contact time or conversion variations. (C) 2003 Elsevier Inc. All rights reserved.