The epoxidation of P-isophorone (3,5,5-trimethyl-3-cyclohexene-1-one) with an amorphous mesoporous titania-silica aerogel and t-butyl hydroperoxide has been investigated. Dominant side reactions were the acid/base catalyzed isomerizations of beta-isophorone to alpha-isophorone and of the product epoxide to 4-hydroxy-isophorone. The acidity of the aerogel was characterized by the epoxide ring opening reaction (to 4-hydroxy-isophorone). Two methods were shown to be efficient in tuning the acidity and the hydrophilic-hydrophobic character of the aerogel, and controlling the product distribution. A catalyst pretreatment in ethyl benzene afforded 98.5 and 93% epoxide selectivities at 75 and 90% peroxide conversions, respectively, without any influence on the oxidation rate. A simple addition of a weak inorganic base (such as NaHCO3 or Na2HPO4) to the reaction mixture and carrying out the reaction in the presence of two solids in an apolar solvent was even more efficient: epoxide selectivities up to 95% were achieved at 90% conversion, with only small changes in reaction time and peroxide selectivity. Moreover, applying other bases the product composition could be varied in a very broad range. 4-Hydroxy-isophorone was the major product with Na2CO3 (65% selectivity at 90% conversion), while alpha-isophorone was formed almost exclusively in the presence of BaCO3 or NaOH. (C) 1999 Elsevier Science B.V. All rights reserved.