Vacuum residuum was upgraded in an ultrasonic reactor of our own design. We found that during sonication in the absence of a hydrogen donor, the viscosity of the vacuum residuum is reduced by 10.98%; without sonication but in the presence of tetralin, it is reduced by 30.9%; and with combined tetralin treatment and sonication, the viscosity is reduced by 39.27%. The synergistic effect of ultrasound and tetralin results in a product with more stable viscosity, higher percentage of light components, lower density and lower pour point than obtained by separate application of these technologies.The amount of vacuum residuum that can be produced in oil refineries is steadily increasing, and may reach 50% of the crude oil [1][2][3][4]. Vacuum residuum is characterized by high density, viscosity, molecular weight, and ash content plus low H:C ratio [5-7], which is an obstacle to refining it on catalytic cracking apparatus for residual crude. Thus there is a need to develop a cost-effective technology for refining vacuum residuum that does not have the limitations of catalytic cracking.Many publications devoted to study of ultrasonic cavitation energy (including [8][9][10][11]) suggest that cavitation technology may be effectively applied to viscosity reduction in vacuum residuum. An ultrasonic generator converts electrical energy to mechanical vibrations, which are propagated within the vacuum residuum as ultrasonic waves [9,12]. The pressure fluctuation leads to generation of microscopic cavities within the vacuum residuum, which expand and violently implode, creating shock waves and thermal energy. The thermal energy and the vigorous

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