Abstract Aldol condensation of benzaldehyde with a variety of carbonyl compounds yielded phenyl alkyl ketones in addition to the expected condensation products over thorium oxide catalyst at 400 °C. The β-hydroxyketone formed underwent both dehydration to yield α,β-unsaturated carbonyl compound and dehydrogenation to yield 1,3-diketone. The 1,3-diketone cleaved into the phenyl alkyl ketone and an acid. Mixed oxide catalysts of thorium oxide with zinc oxide or magnesium oxide were tested for improving the yield of phenyl alkyl ketones. A mixture 80% magnesium oxide-20% thorium oxide was found to be an effective catalyst.