The adsorption and reactivity of thiophene, 2,5-dimethylthiophene, and 3-methylthiophene on Ni(111) have been examined. The saturation coverages of the three molecules are similar, about 0.13 monolayer (ML), and in all cases alkenes are the major hydrocarbon products. On initially clean surfaces, decomposition to sulfur, carbon and hydrogen is the major pathway, but the selectivity to hydrocarbon production can be enhanced by a factor of about 3 by predosing the surface with hydrogen. Sulfur is easily removed from the ring, and C-S bond scission is complete by 150 K. The rate-limiting step in alkene formation is hydrogenation of a highly unsaturated hydrocarbon intermediate. The hydrocarbon intermediates formed are difficult to unambiguously identify but are most likely cyclic structures, retaining the C-4 framework.