Over the past decade, the use of microorganisms to mobilize metals from different spent catalysts has emerged as a promising sustainable approach to metal recyclingRecycling. Herein, we report the preliminary studies carried out for the bioleaching of nickel and molybdenum from a typical catalyst waste from hydrodesulphurization (HDS) operations, employing the bacterium Sulfobacillus thermosulfidooxidans (strain RDB). We applied both direct and indirect bioleaching with respect to pulp density and sulfur addition dosage and observed that indirect bioleaching yielded a better extractionExtraction efficiency of >76% molybdenum and > 94% nickel at 10 g/L pulp density and 15 g/L sulfur addition dosage. The data from this preliminary study will inform future efforts to enhance metal dissolution efficiency on a larger scale, contributing to the realization of a circular economy in the refining industry and beyond.