A process is disclosed for passivating the reactivity of contaminant metals, such as nickel and vanadium, which have been deposited on a catalytic cracking catalyst, by adding to the cracking catalyst a mixture of a calcium-containing material and a magnesium-containing material in a separate reactor in the presence of steam. The preferred calcium-containing material is dolomite and the preferred magnesium-containing material is sepiolite. It is also preferred to include antimony and/or bismuth compounds in the additive mixture.