The mechanisms of the thermolytic decomposition of a series of unsaturated cyclic C-8 hydrocarbons adsorbed on a platinum(111) single crystal surface are described. This study both confirms and extends the results reported by Frei and Campbell which corrected errors in an earlier report from our laboratory on the mechanisms of thermal decomposition seen in this adsorbate system. We find that the dehydrocyclization of cyclooctene, cyclooctadiene (1,3 and 1,5 isomers), and cyclooctatetraene on Pt(111) proceeds through a related set of intermediates and ultimately yields a novel surface-bound bicyclic ring system. As noted by Frei et al., our earlier finding that benzene is produced in high yield is incorrect. The spectroscopic evidence reported here demonstrates that all four of the cyclic C-8 hydrocarbons form a bicyclo[3.3.0]octenyl (or, as more commonly) named, a pentalenyl) intermediate of stoichiometry C8H6. This bicyclic ring system forms via a dehydrocyclization of cyclooctatetraene, which itself is formed in varying yields from each of the less-unsaturated c-C-8 adsorbates. The formation of the pentalenyl species proceeds at low temperatures (<375 K). Data from reflection-absorption infrared (RAIRS) and temperature-programmed reaction spectroscopies show that the pentalenyl intermediate is remarkably stable, persisting on the surface up to at least 450 K. At temperatures above 450 K, this intermediate fragments, liberating hydrogen, and leaving a carbonaceous overlayer. The formation of cyclooctatetraene, and thus the pentalenyl intermediate, proceeds with the highest efficiency for the two dienes and lowest for cyclooctane and cis-cyclooctene. RAIRS data further serve to identify several intermediates formed along the reaction pathway leading to the platinum-bound C8H6 species.