The root cause of hydrotreater feed filter fouling in a bitumen upgrading plant was revealed through a step-by-step scientific investigation. It was first confirmed that the fouling problem was related to a process flow sheet change that introduced a heavy vacuum gas oil (HVGO) stream into the coker combined gas oil (KCGO) stream prior to filtration. Characterization of the foulant and the feed indicated that the fouling reactions are likely oxidative polymerization. Iron naphthenate or naphthenic acid in the HVGO stream could act as a catalyst for such a reaction. A bench-scale oxidation test was carried out to compare the oxygen uptake rates and the C-7-insoluble contents after oxidation in KCGO, KCGO plus HVGO, KCGO plus iron naphthenate, and KCGO plus naphthenic acid streams. While the oxygen uptake kinetics for these samples were similar, the C-7-insoluble contents for KCGO plus HVGO and KCGO plus iron naphthenate increased significantly after oxidation compared to the base case of KCGO. No significant increase of the C-7-insoluble content was observed for KCGO plus naphthenic acid, indicating that it was the iron naphthenate that catalyzed the fouling reactions. Iron naphthenate was a corrosion product in the HVGO stream, which could be eliminated by preventing corrosion in the vacuum distillation unit. The filter fouling problem indeed disappeared after the installation of corrosion-resistant equipment.