The plugging of a fixed bed hydrotreater was determined to be caused by an incompatibility of the mixture of hydrotreater feeds. Insoluble asphaltenes coked a heat exchanger upstream of the hydrotreater, flaked off the heat exchanger, and plugged the catalyst bed. The Oil Compatibility Model and tests were expanded to include oils that contain no asphaltenes. As a result, the most insoluble asphaltenes were determined to be in the fluid catalytic cracker bottoms and the poorest solvent oils were the virgin gas oils. By maintaining the feed composition within the compatibility limits predicted by the Oil Compatibility Model, subsequent plugging of the hydrotreater was avoided.