In the last years, iron poisoning has gained more attention in fluid catalytic cracking operations across the globe. While more and more insights into the mechanisms of its deactivating effects on FCC catalyst particles are available, the early detection of Fe poisoning of FCC catalysts is complicated by the various sources of Fe. This paper describes the use of magnetic susceptibility determinations and digital microscopy for this purpose. The application of magnetic susceptibility shows that the magnetic properties of FCC catalysts change at about 0.30 wt % added Fe indicating that the deleterious effects of the contaminant Fe occur beyond this limit. The combination with inverse gas chromatography and mercury porosimetry determinations confirm the pore restricting effects by Fe poisoning.