The destruction of S-containing molecules is a very important issue in the chemical industry and the control of environmental pollution. The desulfurization of sulfur dioxide and thiophene on molybdenum carbide surfaces, alpha-Mo2C(001) and delta-MoC(001), and on a Mo8C12 nanoparticle was studied with density functional theory. Our study reveals unexpected ligand and steric effects. The Mo8C12 nanoparticle behaves as actively as alpha-Mo2C(001) toward SO2 despite the high C/Mo ratio and C-2 groups. In contrast, SO2 bonds weakly with delta-MoC(001). Spontaneous S-O bond cleavage was observed on both Mo8C12 and alpha-Mo2C(001). Contrary to common assumptions, the C atoms are not simple spectators and play a key role in the energetics for the dissociation Of SO2. In the case of thiophene adsorption, only alpha-Mo2C(001) exhibits a high chemical activity that leads to a spontaneous S-C bond cleavage. The interaction of thiophene with both delta-MoC(001) and Mo8C12 is weak. On the nanoparticle, steric repulsion between the C-2 groups and thiophene overcomes the high reactivity of the Mo atoms in corner or edge sites. Our results illustrate the interplay of ligand and steric effects in nanoparticles of metal compounds.