The decomposition of neopentyl thiol on clean and oxygen-modified Fe(100) surfaces has been investigated under ultrahigh vacuum conditions using temperature-programmed reaction spectroscopy, Auger electron spectroscopy, and high-resolution electron energy loss spectroscopy. On the clean Fe(100) surface upon adsorption at 100 K, the S-H and C-S bonds of the neopentyl thiol cleave, resulting in adsorbed S, H, and neopentyl alkyl fragments, in contrast to the formation of the alkyl thiolate observed on the adsorption of n-alkane thiols on this surface. Upon heating, some neopentyl alkyl fragments react with adsorbed surface hydrogen to evolve neopentane at 260 K. On the oxygen-modified surface, only S-H bond cleavage is observed, and a neopentyl thiolate surface species is formed. No initial neopentane evolution from the oxygen-modified surface is observed upon heating. However, on both surfaces, isobutene and trace amounts of neopentane are seen to desorb at 380 K as a result of an elimination reaction pathway for the majority of the remaining neopentyl hydrocarbon species. Small hydrocarbon fragments remain on the surface until they finally decompose to leave atomic carbon and sulfur on the iron surface at 500 K.