The epoxidation of beta-isophorone with TBHP and an amorphous mesoporous titania-silica aerogel, containing 20 wt% TiO2, has been investigated. The nature of side reactions and the possibility of suppressing them by modification of the catalyst with bases was studied. The as-synthesized (unmodified but calcined) aerogel afforded only moderate selectivity to epoxide (36-87%) in the temperature range of 343-373 K. 4-Hydroxy-isophorone and alpha-isophorone were the major by-products, formed in isomerization reactions of product and reactant, respectively, and catalyzed by acidic sites present on titania-silica. The selectivity to epoxide could be improved by pretreatment of the aerogel with neutral or weakly basic alkali or alkaline earth metal salts. Treatment of the aerogel with aqueous NaOAc followed by recalcination at 873 K afforded 94% selectivity at 90% peroxide conversion. Application of stronger bases was detrimental to the epoxidation activity and selectivity.We propose that the improved selectivity is due to ion exchange between the weakly basic additive and the surface Bronsted sites (silanol groups). Neutral salts are preferentially anchored to the polar (acidic) surface sites and reduce their accessibility for the more bulky and less polar reactant. The detrimental effect of strong bases is partly due to excess basicity of the catalyst after pretreatment, as the side reactions are also catalyzed by bases. Besides, prolonged treatment with NaOH partially hydrolyzed the Ti-O-Si bonds and restructured the material (titania microdomains in the silica matrix), as evidenced by FTIR and Uv-vis spectroscopy. The study of the epoxide ring opening reaction demonstrated the limited relevance of ex situ acidity measurements for predicting the role of acid sites in complex epoxidation reactions.