Abstract : This work focuses on the instability phenomena occurring during vacuum residue hydroconversion. At high level of residue conversion, carbonaceous sediments (sediments) are formed and have detrimental effects on the industrial units during the hydroprocessing operations. The aim of this work is investigate the influence of the catalyst used inhydroconversion process on the sediments formation. Modified catalysts have been prepared by sodium and fluorine deposition on a NiMo/Al2O3 reference catalyst. After having been characterized, the catalysts have been tested in a perfectly stirred batch reactor. Catalytic tests have been performed at high temperature (430°C) in industrial conditions and at lower temperature (390°C) in order to favor catalytic reactions rather thermic reactions. The catalytic tests at the two temperatures show that the amount of sediments is two times lower with F-NiMo catalyst. This is explained by the improvement of asphaltenes conversion due to an increase of catalyst acidity and hydrogenation reactions. At 390°C, coking with F-NiMo catalyst is reduced so that porous volume is higher. The temperature effect on effluents stability has also been studied. Results show that the amount of sediments is twice lower at 390°C for a same catalyst. The increase of instability at 430°C is attributed to more condensed and aromatic asphaltenes and resins dealkylation which increase selfaggregation tendency of asphaltenes.