In a fluid catalytic cracking process for cracking hydrocarbons with a zeolitic cracking catalyst and in the absence of added hydrogen, the desired value of CO.sub.2 /CO in the flue gases from the regenerator is achieved by including in the catalyst a combustion-promoter metal such as platinum, palladium, rhodium, ruthenium, iridium, osmium or rhenium, and, additionally, contacting the catalyst with a metal-free chlorine or bromine compound.