Before subjecting the raw material to cracking in the presence of catalysts at temperatures 200°-450° under pressures of 5-150 atm., it is given a very mild dehydrogenating and isomerizing treatment in the presence of catalysts so that virtually no cracking into low boiling products results. Example: A heavy gasoline b. 100°-200° obtained through the reduction of CO is passed over a catalyst consisting of active alumina with 10% of molybdenum oxide at 430° and normal pressure. The product has virtually the same boiling range as the raw material. It contains about 50% of olefins and has an octane number of 60. It is then passed over artificial Al silicate at 460° at a flow rate of 1 kg. oil per 1. of catalyst per hr., whereby 80% by weight of the raw material is converted into a gasoline b. up to 200° (38% up to 100°) and having an octane No. of 72 motor method.