The structures of nickel deposits on sulfided CoMo/Al2O3 hydrodemetallation catalyst surface were characterized by using scanning transmission electron microscopy and high-resolution transmission electron microscopy techniques to study the interaction between the deposits and catalytic components. The deposits were found in crystallite form of nickel sulfide (Ni7S6) on the catalyst surface. Within the crystallites cobalt is uniformly distributed. Molybdenum is only partially associated with the nickel deposits as a segregated surface layer of molybdenum sulfide (MoS2). For crystallites smaller than about 15 nm, the extent of segregation decreases. About 75% of the molybdenum is not directly associated with nickel deposits. Extensive cover-up of molybdenum by nickel deposits was not observed, and is not the main reason of catalyst deactivation. It was found that nickel deposits migrated towards cobalt sites even though molybdenum sites were the active sites for the hydrodemetallation reaction. Structure affinity favored the formation of solid solution between cobalt and nickel sulfides, which was the driving force for the association between nickel and cobalt on the catalyst surface. Microscopic characterization showed that deposition of nickel sulfide on the catalyst surface enhanced the mobility of the catalytic components MoS2. Coalescence of MoS2 leads to significant reduction of accessible molybdenum sulfide sites, and is the main reason for the deactivation of hydrodemetallation catalyst.