The formation of MoSxCy compounds has been observed on/in the surface of molybdenum-sulfide catalysts during the hydrodesulfurization (HDS) process, and it is a major factor for determining the activity of molybdenum-carbide catalysts. Density functional theory (DFT) was employed to investigate the adsorption of sulfur and sulfidation of transition metal carbides from groups 4-6 in the periodic table, including extended surfaces [MC(0 0 1) (M = Ti, V, Mo, Ta)], nanocrystals [M14C13 (M = Ti, V, Mo)] and metcar [M8C12 (M = Ti, V, Mo)] nanoparticles. It was found that with increasing carbon/metal ratio, the reactivity of the metal carbides towards sulfur decreased in the sequence: Mo2C > M14C13, M8C12 > MC(0 0 1)- In terms of sulfidation, M8C12 and MC(0 0 1) display a stronger resistance than M14C13. The presence of comer or edge sites in the M14C13 nanocrystal favors the formation of MoSxCy, compounds. Following Sabatier's principle, our results suggest that flat MC(00 1) surfaces are too inert to catalyze HDS reactions, while M14C13 is too active to resist the sulfidation that leads to degradation of the carbides. For reactions involving sulfur and sulfur-containing molecules, nanoparticles adopting the special geometry of metcars should display a better catalytic activity than the corresponding bulk materials and carbide nanoparticles that have a cubic-based structure like nanocrystals. Indeed, DFT calculations indicate that Ti8C12 and Mo8C12 are good catalysts for the HDS of thiophene. (c) 2005 Elsevier B.V. All rights reserved.