The structures and reactivities of various cyclic C-7 hydrocarbons (cycloheptane, cycloheptene, 1,3-cycloheptadrene, and cycloheptatriene) adsorbed on Pt(lll) have been examined by means of temperature-programmed reaction, reflection-absorption infrared, and Auger electron spectroscopies. At temperatures below 200 K, cycloheptane binds weakly to the surface and exhibits vibrational mode softening consistent with the presence of agostic C-H ... M interactions. In contrast, the cyclic alkenes cycloheptene, 1,3-cycloheptadiene, and cycloheptatriene all bind strongly (and largely irreversibly) to the surface principally by means of interactions with their pi-systems. The chemistry of these adsorbates at higher temperatures has been mapped out in considerable detail. For cycloheptane, most simply desorbs but about 30% is dehydrogenated; interestingly, cycloheptene is not an intermediate in this process. Cycloheptene first loses two hydrogen atoms to give what we propose is an eta(3),eta(1)-C7H10 product. 1,3-Cycloheptadiene first loses two hydrogen atoms to afford an eta(5)-cycloheptadienyl product, which dehydrogenates further to give surface-bound cycloheptatriene in (apparently) high yield. Between 325 and 375 K, all of the cyclic C-7 hydrocarbons dehydrogenate further and eventually yield a planar eta(7)-cycloheptatrienyl (C7H7) species on the surface, although for cycloheptene this conversion is inefficient. The thermolytic decomposition of the eta(7)-C7H7 species begins at about 425 K and affords surface-bound hydrocarbons that are largely undetectable by RAIR spectroscopy. At higher temperatures (>800K), these hydrocarbons are completely dehydrogenated to form a carbonaceous overlayer. Ring contraction products, such as surface-bound benzene, are not observed as intermediates in any of the thermolytic reactions. The RAIR spectra of the eta(5)-cycloheptadienyl and cycIoheptatriene adsorbates both contain an unusual low-frequency vibrational band near 2770 cm(-1). This band is not indicative of an agostic C-H .. M interaction, bur is instead the signature of a methylene C-H bond that has been perturbed by hyperconjugative drain of electron density into the adjacent pi-system.