a method for passivating a catalyst used to crack hydrocarbon feedstock to lower molecular weight products in a reaction zone is disclosed where the feedstock contains at least one metal contaminant selected from the class consisting of nickel , vanadium and iron and where the contaminant becomes deposited on the catalyst such that at least a major portion of the total of said metal contaminant deposited on the catalyst comprises only one of the metal contaminants . the method comprises monitoring the composition of said metal contaminant on the catalyst , adding a predetermined amount of at least one of said metal contaminants not present as the major contaminant on the catalyst , and passing the catalyst from the reaction zone through a reduction zone maintained at an elevated temperature for a time sufficient to at least partially passivate said metal contaminant . cross - reference to related applications this application is related to u . s . patent application , ser . no . 108 , 395 filed on even date herewith .