Due to environmental concerns much effort has been devoted in recent years to the development of processes to rejuvenate and reuse metal-fouled spent catalysts from residue hydroprocessing units. As a part of an extensive research program on the subject. we report in this paper about the extraction of the deposited vanadium from the spent catalyst using ferric nitrate-oxalic acid reagent system and its effect on catalyst's surface area and activity recovery. A comparative assessment of different modes of ferric nitrate addition to oxalic acid, namely, (i) continuous addition, (ii) batch addition, and (iii) successive addition, on the leaching efficiency was made. The results revealed that addition of ferric nitrate to oxalic acid enhanced its leaching activity remarkably, and continuous mode of addition was more effective than the others. The improvements in the surface area and pore volume of the catalyst were better and about 85% of the HDS activity was recovered for the catalyst leached with ferric nitrate-oxalic acid reagent system in a continuous addition mode. Removal of the coke from the leached catalyst increased the HDS activity further from 85 to 95% of fresh catalyst activity. The unleached vanadium remaining in the catalyst was not concentrated at pore mouths, but was well distributed within the pellet. The enhanced leaching of vanadium in the continuous addition mode of the oxidizing reagent was explained in terms of a synergistic mechanism involving oxidation and complex formation reactions of the low-valent vanadium sulfide species during treatment with the ferric nitrate-oxalic acid reagent system. (C) 2003 Elsevier Science B.V. All rights reserved.