We have examined steam dilution in the cracking of 2-methylpentane on USHY at 400, 450, and 500 degrees C. Steam appears to act as a more "intrusive" diluent than the diluents we have previously examined, i.e., N-2, H-2, CO2, and CO. We find that at 400 degrees C (and probably below this temperature), a small amount of H2O has a strong promoting effect on the rate of the isomerization reaction which accompanies cracking. This effect is so strong at small dilution ratios that, rather than reducing the rate of overall conversion (by diluting the reactants), steam actually increases the overall conversion rate. However at 450 degrees C and above, even low levels of steam dilution reduce the overall rate of conversion, as we might have expected. Due to this temperature dependence of the dilution effect, steam dilution reduces the apparent activation energy of the overall reaction, making the rate of the cracking reaction less sensitive to temperature changes in the presence of steam. Higher dilution ratios invariably reduce the rate of conversion with the result that at the low temperature there is a dilution ratio which yields a maximum rate. It is this maximum rate that is higher than the rate of the undiluted reaction. Here we present quantitative data on these effects and offer a possible interpretation of the mechanism behind it in order to provide a framework for the discussion and utilization of this interesting phenomenon.