The thermal chemistry of 1,3-diiodopropane, 1-chloro-3-iodopropane, 1-iodopropane, 1-chloropropane, 3-chloropropene, propene, and cyclopropane on Ni(100) surfaces has been studied under ultrahigh vacuum conditions by using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Cyclopropane, propene, propane, and hydrogen are all produced by thermal activation of the diiodopropane at low coverages, and iodopropane and molecular desorption are also seen at higher coverages. In contrast with this, only cyclopropane, propene, and chloropropane-but no propane-were observed after thermal activation of 1-chloro-3-iodopropane. Both I 3d and Cl 2p XPS spectra suggest that the adsorption of the dihaloalkanes is molecular below 100 K and that at low coverages both halogen atoms interact directly with the surface. They also point to the fact that the C-I (C-CI) bonds break between 140 and 180 K in steps that most likely lead to the generation of a three-carbon metallacycle on the surface. These metallacycles may then undergo either intramolecular coupling to form cyclopropane or a dehydrogenation step to generate propenyl groups on the surface, and propenyl moieties, produced either by thermal activation of the metallacycle mentioned above or directly via the surface decomposition of 3-chloropropene, dehydrogenate to propene. Finally, some iodopropyl species are generated at high 1,3-diiodopropane coverages, and those incorporate a surface hydrogen to produce iodopropane (which either desorbs or undergoes a second C-I bond scission to give propyl species, the same as when starting directly with 1-iodopropane), 1-Chloro-3-iodopropane loses an iodine atom first, and some of the resulting 3-chloropropyl yields 1-chloropropane and 3-chloropropene, while the rest reacts further to produce cyclopropane and propene, as in the diiodopropane case. Nevertheless, very little cyclopropane is detected at high 1-chloro-3-iodopropane coverages or in the presence of coadsorbed hydrogen, presumably because the formation of metallacycle species is not favorable in that case. Finally, 1-chloropropane mostly desorbs molecularly and does not yield any hydrocarbon products.