Polished cross sections of a series of spent catalysts used in processing of Athabasca bitumen and other feeds were examined by optical microscopy. Some of the samples contained distinct domains of high reflectance, indicating the presence of coke of higher aromaticity than the surrounding matrix. These domains had a mean diameter of ca. 26 mu m. Fluorescence due to adsorbed feed components and anisotropy due to mesophase formation were found in some samples but not in all. The observations on the series of samples were consistent with the adsorption of basic compounds on acidic domains in the gamma-alumina, followed by coking of these domains. In samples of catalysts used to process residual feed, the background matrix eventually developed aromatic coke deposits, and the heterogeneity was no longer visible.