The condensation reaction of acetone on alumina and acid zeolites has been followed by FTIR. Under identical conditions, the reaction rate is faster on alumina, and the condensation goes beyond the formation of mesityl oxide. Zeolites without nonframework aluminum are poor catalysts. On HZSM-5 the reaction is about two orders of magnitude slower than on USY at 105 degrees C. From these data, it appears that Lewis sites, even if they bound acetone less energetically than Bronsted sites, are responsible for the activation of the molecule. On alumina, the reaction would take place between gas phase acetone and acetone adsorbed on Lewis sites. On zeolites with nonframework aluminum and, thus, with Lewis sites, the reaction would involve acetone molecules adsorbed on Bronsted and Lewis sites, the activation occurring on the Lewis site.