Synchrotron-based high resolution photoemission has been used to study the adsorption and decomposition of thiophene on clean Mo(110) and carbide- or sulfide-modified molybdenum (MoC x and MoS x , respectively). Thiophene adsorbs molecularly on Mo(110) at 100 K. By 180 K, upon desorption of the thiophene multilayer, chemisorbed thiophene and its decomposition products (S and C x H y fragments) coexist on the Mo(110) surface. At 250 K, no CS bonds are left. There is a continuous transformation of the C x H y species into atomic C at temperatures between 300 and 600 K. The chemistry of thiophene on clean and carbide-modified molybdenum is very similar. In contrast, MoS x is less reactive than Mo(110) or MoC x , even after introducing a large number of sulfur vacancies in the sulfide or after promoting it with nickel. Thiophene does not dissociate on MoS x or Ni/MoS x . The differences in chemical reactivity between MoS x and MoC x correlate well with changes in the electronic properties of Mo in these systems.