Other researchers have reported (J. Am. Chem. Soc. 1993, 115, 2044; J. Phys. Chem. 1994, 98, 2952) that cyclooctene dehydrogenates on Pt(111) to stable adsorbed cyclooctatetraene, which then undergoes ring contraction to produce benzene but without any calibrated measurements of the yield. Here, quantitative thermal desorption mass spectrometry (TDS) and bismuth postdosing thermal desorption mass spectrometry (BPTDS) are used to investigate the conversion of cyclooctene to benzene on the Pt(111) surface. Our results show that although benzene is formed, it is a very minor product, corresponding to less than 2% of a monolayer (including both adsorbed and gas-phase benzene). Most of the adsorbed cyclooctene either desorbs intact at low temperatures (similar to 10%) or simply dehydrogenates (similar to 90%), ultimately to surface carbon by 800 K, but without going through adsorbed benzene as an intermediate. Stable intermediates with stoichiometries C8H12 and C8H6 are identified by TDS to be present at 350 and 430-560 K, respectively, but BPTDS shows that neither of these correspond to a simple molecularly adsorbed state of a stable gaseous molecule. During the conversion between these two species, however, cyclooctatetraene is produced transiently at 430 K, suggesting that both of these species still have an intact C-8 ring.