A catalyst, useful in the preparation of isoolefins and containing 0.1 to 20% by mass of an alkali metal oxide, an alkaline earth metal oxide and mixtures thereof; 0.1 to 99% by mass of aluminum oxide; and 0.1 to 99% by mass of silicon dioxide, is prepared by a) treating an aluminosilicate with an aqueous alkali metal salt solution, an alkaline earth metal salt solution and mixtures thereof, under acidic conditions, to obtain a treated aluminosilicate; and b) calcining the treated aluminosilicate, to obtain the catalyst.