The diffusion rates of feedstocks and intermediate products in fluid catalytic cracking (FCC) catalysts have relevance for the product selectivities obtained from the conversion of feedstocks in the FCC process. A simple and robust method was developed to determine the diffusion of probe molecules in such catalysts by means of inverse gas chromatography. This procedure is described in detail, and its relevance in FCC is illustrated by several examples. It is shown that catalytic properties of FCC catalysts can be rationalized with the diffusion coefficient parameter in cases where traditional analytical tools (nitrogen sorption and X-ray diffraction) reach their limits.