The adsorption and thermal reactivity of c-C 5H 8 on a Pt(111) surface has been studied by quantitative thermal desorption spectroscopy, high resolution electron energy loss spectroscopy and work function changes, and the bonding configurations and reactivity compared and contrasted with the linear olefins. At < 250 K, associative π-donor-acceptor bonding in the Dewar-Chatt-Duncanson sense has been rejected. Instead, the vibrational data are more consistent with extensive sp 2 → sp 3 rehybridization of the olefinic bond leading to a bonding configuration designated η 2(di- σ)- c-C 5H 8. Steric considerations suggest that there is a tilting of the C 5 ring which manifests itself in a soft CH stretch near 2750 cm −1 attributed to additional hydrogen-bonding interaction with the surface. At 250–300 K the partial dehydrogenation pathway of the linear olefins to an adsorbed alkylidyne is inhibited by the inherent stability of the C 5 ring. Instead, three hydrogen atoms from η 2(di- σ)- c-C 5H 8 are dissociated to produce a new adsorbed species identified as a pentahapto-cyclopentadienyl, η 5-Cp, which is covalently π-bonded to the surface in a manner analogous to the inorganic metallocenes like ferrocene, FeCp 5. Under these conditions, η 5-Cp essentially saturates the surface with a coverage estimated at 1.5 ± 0.5×10 10 molecules cm −1. With the surface saturated with η 2(di- σ)- c-C 5H 8 the onset of hydrogen desorption at 250 K is accompanied by the desorption of excess parent c-C 5H 8. It is proposed that c-C 5H 8 is sterically displaced from the surface by the greater Van der Waals dimensions of the more strongly bound η 5-Cp. c-C 5H 8 is not evolved from submonolayer coverages. Desorption of both hydrogen and c-C 5H 8 occurs with a common reaction-limited onset at 250 K. However, whereas c-C 5H 8 desorption occurs with a peak at 285 K, hydrogen may, with increasing conversion, be accommodated on the surface and subsequently desorbs in a β 2 desorption-limited step at 305 K. η 5-Cp is stable to 480 K when it dehydrogenates leaving, at 750 K, carbon on the surface. Decomposition is complex with the vibrational data suggesting a methylidyne stable to 600 K and a MCHCHM species which is somewhat less stable.