The effect of steam dilution in shaping the selectivity of 2-methylpentane cracking has been studied at 400, 450, and 500 degrees C using a USHY zeolite. The results show that the presence of steam enhances isomerization of the feed and suppresses the production of cracked products. This and other evidence leads us to believe that steam dilution enhances the chain propagating reactions in the mechanism of catalytic cracking. At the same time, steam inhibits chain termination by desorption, The net result is that the presence of steam increases kinetic chain length and the paraffin-to-olefin ratio, The effects of steam dilution are surprisingly strong at lower temperatures but become less significant as temperature rises. All these effects are interpreted in terms of their influence on the elementary steps of the chain mechanism and are principally rendered in terms of a new measure : the reaction path probabilities (RPPs). These describe the likelihood that a feed molecule will be converted by a given reaction. Probabilities of all. the reactions of the various ions present in the chain mechanism of 2-methylpentane cracking are quantified by formulating appropriate secondary functions using the RPPs derived from our experimental data. These various "internal" probabilities are used to quantify just how likely are each of the many alternative reactions of a given ion, as reaction conditions change. They allow us to track the effect of changes in reaction conditions, or in catalyst formulation, in unprecedented detail. Moreover, they give a detailed picture of the behavior of elementary processes in this reaction, We present and discuss examples of the quantitative information which our methods make available, and the mechanistic interpretations which how from this information, In particular, we apply these measures in a study of the effects of steam when it is added to cracking 2-methylpentane in various proportions. It appears that catalytic cracking is much more susceptible to the effects of dilution than has previously been suspected.