Abstract: Various fractions of petroleum, including residual oils and crude oils, are catalytically converted to produce gaseous olefins, especially propylene and butylene, in fluidized or moving bed or transfer line reactors with solid, acidic catalysts in the presence of steam at a temperature of 500.degree. to 650.degree. C. and a pressure of 1.5.times.10.sup.5 Pa to 3.times.10.sup.5 Pa, with a weight space velocity of 0.2 to 20 hr.sup.-1 and catalyst-to-oil ratio of 2 to 12. Spent catalyst is continuously removed from the reactor to a regenerator where the coke is burned off and the hot catalyst is returned to the reactor. In a comparison with conventional catalytic cracking and tubular furnance pyrolysis processes, it is found that the process of the present invention produces more propylene and butylene. The total yield of the process of the present invention is about 40 percent by weight of the feedstock.