Adsorbed states of cyclopentene on Si(100)(2 x 1) have been investigated by means of low energy electron diffraction (LEED), valence and Si 2p photoelectron spectroscopy (PES) using synchrotron radiation, and high-resolution electron energy loss spectroscopy (HREELS). Valence and Si 2p PES results indicate that the dangling bonds of the Si(100)(2x1) dimer and the pi-bond of cyclopentene interact with each other, resulting in the formation of Si-C bonds. The valence electronic structure of adsorbed cyclopentene is similar to that of gas-phase cyclopentane. In addition, the HREELS spectra of cyclopentene on Si(100)(2x1) resemble the vibrational spectra of free cyclopentane and the cyclopentene di-sigma bonded to Pt(111). Therefore, we conclude that cyclopentene is di-sigma bonded to the Si(100)(2x1) surface and the molecular structure is similar to that of cyclopentane (all the carbon atoms are in sp(3) state); the possible adsorbed structures are discussed. At saturation, 76% of the dimers are reacted, indicating that the unreacted dimer sites exist owing to the steric hindrance of adjacent molecules.