The invention relates to the use of compounds of formula:[M(II).sub.1-x M*(III).sub.x (OH).sub.2+x ].mH.sub.2 O (I)in which M(II) represents a divalent cation chosen from the group consisting of nickel, zinc, cobalt, magnesium or the mixture of two or more of the abovementioned metals; M(III) represents a trivalent cation, such as aluminum, gallium, iron or chromium, with the exception of the compound of formula (I) in which M(II) is magnesium and M(III) is aluminum, x is between 0.20 and 0.33 and m.ltoreq.1; in the preparation of .beta.-hydroxycarbonyl and/or .alpha.,.beta.-unsaturated carbonyl compounds.