The fast transient temporal analysis of products (TAP) reactor has been successfully applied as an alternative tool for investigating the hydrodynamic and catalytic properties of commercial FCC catalysts: (i) diffusion of isooctane as a probe molecule within the various interfaces between the gas phase and the cracking acid sites (matrix mesoporosity and zeolite microporosity), (ii) applied cracking mechanism: unimolecular (protolytic via alkanium ions), or bimolecular (via carbenium ions) or a combination of both, hydride transfer and side reactions. The relationship between the above parameters and the state and composition of the tested catalysts is discussed.