The thermal chemistry of 1-methyl-1-cyclopentene (1MCp(-)) and methylene cyclopentane (MeCp) was investigated on clean and hydrogen-and deuterium-predosed Pt(111) single-crystal surfaces by temperature-programmed desorption and reflection-absorption infrared spectroscopy. It was found that MeCp isomerizes easily to 1MCp(-) but that the reverse reaction does not occur, at least under our experimental conditions. The MeCp to 1MCp(-) isomerization is aided by the presence of coadsorbed hydrogen, and occurs through the formation of a common 1-methyl-1-cyclopentyl ( 1MCp- Pt) surface intermediate; that intermediate then undergoes beta-hydride elimination selectively at the ring position to form the 1MCp(-) product. In addition to this isomerization, both 1MCp(-) and MeCp also dehydrogenate on the Pt( 111) surface to form a methylcyclopentadiene species, at 325 and 350 K, respectively. A small amount of benzene desorption is detected above 500 K with both reactants, indicative of a ring enlargement reaction. No evidence for the activation of any of the allylic hydrogens was obtained in either molecule.