Iron poisoning is a well-known problem for fluid catalytic cracking catalysts. To better understand this issue, several characterization techniques were used to examine a catalyst sample from a commercial unit. Under SEM-EDX, most cross sections of the catalyst particles exhibited iron nodule rims enriched with Fe, other contaminant metals, and Si. Scanning TEM X-ray mapping further confirmed that, compared to the inside of a catalyst particle, the surface layer was enriched in Si and contaminant metals, while depleted in Al. This catalyst had high frequency-dependent magnetic susceptibility, suggesting that some or all of the iron-bearing species should be in the nanometer size range. In TEM, the nodules were found to consist of Fe-bearing nanoparticles, imbedded in an amorphous matrix. The nanoparticles were also enriched in Mg, Ni or Ca, while the matrix was dominated by Si and minor Al. Selected area electron diffraction pattern indicated that these iron enriched nanoparticles had the spinet structure. These results should shed light on iron poisoning issues in FCC units. (C) 2018 Elsevier Inc. All rights reserved.