Spent hydrotreating catalysts are not inert to air, humidity and temperature, as the combination of these factors may gradually leach metals from the catalyst causing contamination of land and groundwater. In order to investigate the combined effect of ambient variables on metals leaching, a spent hydrotreating catalyst is placed in a tin can heated at 40-50 degrees C and then treated for 1000 h under a continuous flow of wet air. Said spent catalyst contains 10.68 wt.% V, 0.43 wt.% Fe, 2.67 wt.% Ni and 3.78 wt.% Mo as well as 14.7 wt.% coke and 19.4 wt.% sulfur, the latter basically in the form of metallic sulfides. During the leaching experiment, samples of spent catalyst are taken at different times to track the evolution of metals and coke/sulfur content. At the end of the test, even though relatively low temperatures are used, Ni is fully removed, Mo and V are partially extracted (around 25% for each metal) while the Fe content increases. Practically no leaching of metals is observed on the sulfur/coke-free fresh catalyst subjected to an identical experimental procedure. The presence of sulfur and coke on the spent catalyst clearly enhances metals leaching. The sulfur oxides produced from the oxidation of metallic sulfides with air, in combination with water, results in the formation of acidic species, which have a strong positive influence in leaching metals from the spent catalyst. (C) 2012 Elsevier Ltd. All rights reserved.