The influence of H2S (0-559.6 kPa) on Maya crude hydrotreating is investigated in an integral fixed bed up-flow micro reactor. The added H2S inhibits hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) while asphaltene conversion (HDAs) remained almost unaffected. On the other hand, a promotional effect is found for hydrodemetallization (HDM). The observed variation in HDS and HDM conversions suggests a dual nature of catalytic sites particularly at high partial pressure of hydrogen sulfide. The promotional effect for HDM may be interpreted in terms of adsorption of metal-porphyrins on Bronsted acid sites (sulfhydryl group), which enhance hydrogenation of metal-porphyrins and convert them into the corresponding metal-chlorin structure in a first step of reaction. The final step in the HDM is essentially hydrogenolysis (metal-nitrogen) and requires the presence of an anionic vacancy (CUS). The conversion of asphaltene is also depending on the acidic nature of sulfided catalyst that is remaining either uninhibited or slightly enhanced with H2S. (C) 2006 Elsevier Ltd. All rights reserved.