Catalyst for steam cracking reactions consisting of pure mayenite having the general formula:12CaO.7Al2O3having an X-ray diffraction spectrum as indicated in Table I, obtained with a preparation process comprising the following steps: dissolution of salts containing calcium and aluminum with water; completing of the dissolved salts by means of polyfunctional organic hydroxyacids; drying of the solution resulting from the completing in order to obtain a solid precursor product; calcination of the solid precursor product at a temperature ranging from 1300 to 1400null C. for at least two hours.