Detailed information gathered on the cracking of five hexane isomers has revealed major differences in the mechanisms of cracking in the individual isomers and suggests the possibility that synergistic effects play a role in the cracking of mixtures of hydrocarbons. We find that both n-hexane and 2,2-dimethylbutane do not crack via the chain process to any significant extent, presumably due to the absence of tertiary hydrogens in both molecules. On the other hand, 2,3-dimethylbutane, with two tertiary hydrogens, cracks readily via a chain process. The two monomethyl pentanes illustrate the fact that tertiary hydrogens are not all the same in terms of reactivity. 3-methylpentane has a much more reactive hydrogen than 2-methylpentane, with the result that more of this feed is converted by chain processes. However, the overall rate of conversion is higher in 2-methylpentane and lower in 2, 3-dimethylbutane. It seems that, when it comes to maximizing the overall rate of reaction, the dominant influence is the rate of initiation by protolysis. In this work we examine the initiating protolysis reactions of the partent hexane molecules. These reactions start the chain processes which, in various measures, are responsible for the observed overall conversion in "catalytic cracking." We find that the rates and modes of the various protolysis reactions are profoundly different in the five isomeric hexanes. In discussing this fact we raise the issue of the significance of "test reactions" I in catalyst evaluation.