Representative samples of regenerated hydrotreating NiO-MoO3/Al2O3 catalyst were investigated in order to define the main reasons for activity decline, observed after restarting the industrial reactor. By complex structural and textural characterization, followed by surface chemical analysis, it was shown that pronounced sintering and structural changes take place in a fraction of catalyst bed, particularly in the upper layer. The intermediary presence of MoO3 in the liquid phase was assumed to be the cause of the intensive sintering during regeneration, which was followed by loss of active component. Restructuring and segregation of the active phase in hot spots, resulting from blocking of a part of the catalyst bed, are supposed to be the main causes for changes taking place later on during regeneration.