Process changes in a bitumen-upgrading plant resulted in an unexpected fouling problem on the feed filters to hydrotreater units. This work was undertaken to investigate the nature of the foulant and to determine its origin. Samples of foulant were extracted directly from a filter, fractionated, and characterized by a number of complementary techniques. These results were then compared to similar data from various gas oil fractions found in the feed. We concluded that the fouling was caused by oxidation of a nitrogen-rich pyrrole fraction present in gas oils.