Accumulation of fine particles by packed-bed catalytic reactors causes increased pressure drop, sometimes causing premature shut down, Deposition in a reactor, i.e., filtration, is enhanced when the particle suspension is unstable. A model system of 0.68 mu m of kaolin in light gas oil was selected to study deposition at hydrotreating conditions. The surface of the kaolin was modified by adsorbing polymeric asphaltene molecules. Particle suspensions were circulated through a packed bed of either glass beads or catalyst in a batch reactor at 375 degrees C. The filtration mechanism switched from cake filtration to deep-bed filtration when kaolin was pretreated with asphaltene. Deposition of kaolin particles in the bed decreased with increasing asphaltene concentration. These results are consistent with steric stabilization of colloids by adsorbed polymers. Hydrotreating of the liquid phase enhanced desorption of the asphaltene polymer from particle surfaces and increased particle flocculation and deposition along the length of the reactor bed.