Understanding of the process of hydrogen redistribution in catalytic cracking has long been recognized as essential to understanding of the coking processes responsible for catalyst decay. Hydrogen redistribution is also thought to be behind the puzzling excess of paraffins over olefins that has been noted by several authors [1-3], and to hold the key to determining the selectivity of cracking reactions. If the transfer of hydrogen during catalytic cracking can be understood and quantified, much of the detail of the overall process should become clear.