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The thermal chemistry of 1,3-diiodopropane on a Pt(111) single-crystal surface was investigated by temperature programmed desorption (TPD) and reflection−absorption infrared spectroscopy (RAIRS). It was found that the first decomposition steps of the chemisorbed diiodo compound are the scissions of its C−I bonds, and that that takes place sequentially and results in the initial formation of an iodopropyl surface intermediate which subsequently decomposes to a C3 metallacycle. Upon further heating of the crystal, the metallacycle intermediate dehydrogenates via a β-hydride elimination step to form an allylic moiety. This allylic species then hydrogenates to propene, a product that subsequently desorbs in two different temperature regimes (around 240 and 330 K) or dehydrogenates to surface propylidyne. The metallacyclic surface intermediate displays additional chemistry at high coverages. Specifically, some of the hydrogen made available by β-hydride elimination from a few of the C3 metallacycles is incorporated into other metallacycle moieties to form a 1-propyl intermediate. This 1-propyl group then undergoes β-hydride elimination to propene or hydrogenates further to propane. Finally, the propene produced via 1-propyl dehydrogenation desorbs molecularly or hydrogenates back to either 1- or 2-propyl intermediates. When 1,3-diiodopropane is coadsorbed with deuterium, the dynamic propyl−propene interconversion results in extensive H−D exchange, yielding all possible isotopomers of propene and propane. The implications of the overall complex hydrogenation−dehydrogenation mechanism identified in this study to catalytic systems are discussed.