Preparation of isophorone

A vapor phase aldolization process for producing isophorone and mesityl oxide from acetone which comprises contacting said acetone with a calcined magnesium/aluminum mixed oxide catalyst derived from an aqueous mixture consisting of from about 0.1 to about 5 parts by weight of a water-insoluble magnesium carbonate selected from the group consisting of magnesium carbonate hydroxide pentahydrate, anhydrous magnesium carbonate, magnesium carbonate monohydrate, and magnesium carbonate hydroxide trihydrate, per one part by weight of a water-insoluble aluminum hydroxide carbonate hydrate, said catalyst being calcined at a temperature of from about 300.degree. C. to about 500.degree. C. and from about 1 to about 10 parts by weight of water per one part of the total weight of said water-insoluble aluminum and magnesium compounds employed, and recovering an aldolization product of isophorone, mesityl oxide and unreacted acetone.