The epoxidation of isophorone with TBHP over unmodified and dimethylaminopropyl-modified titania-silica aerogels is studied. Organic modification diminishes the epoxidation activity of titania-silica, but it also eliminates the activity in acid-catalyzed side reactions, such as epoxide and peroxide decomposition. Addition of MeOH to an unmodified aerogel has a similar effect. These catalytic experiments suggest that (i) electron donation by the alkylamino groups or MeOH to the Ti active site (Lewis acid) suppresses its acidity and thus the rate of acid-catalyzed oxidation and isomerization reactions, and (ii) the surface Si-OH groups (Bronsted sites) of titania-silica are poor catalysts for epoxide decomposition.