The nuclear microbeam analysis techniques of proton-induced X-ray emission (PIXE) and nuclear reaction analysis (NRA) have been used to probe the distribution of carbon and other elements in used fluid catalytic cracking catalyst particles. The catalyst was sampled from the stripper standpipe in one of Shell＇s commercial riser FCC units processing feedstocks containing up to 4 wt% Conradson carbon residue (CCR), PIXE and NRA gave quantitative information on the metal and carbon distributions. Samples were embedded in waterglass (Na2O . SiO2) to overcome the problem of carbon contamination from conventional embedding materials. Elemental line scans and images were obtained with a spatial resolution of a few microns and sufficient sensitivity to allow quantification of ah elements of interest. Correlations were found between the spatial distribution of Ti, V, and, to a lesser extent, Fe. Ni was found to be confined to the outermost 10 mu m or so of each particle. The degree of correlation appeared to reduce with particle age. Despite the high CCR content of the feedstock, it was found that carbon was uniformly distributed through the particles, indicating that even the bulkier molecules present in the feed are cracked on the external surface of the FCC particle.