Adsorption of 2-butyne (CH3C=CCH3) on a Si(100)-(2x1) surface was examined using scanning tunneling microscopy (STM), Cs+ reactive ion scattering (RIS), and density functional theory calculations. STM and RIS investigations show that 2-butyne chemisorbs on the surface as a molecule without dissociation. In STM images the adsorbed 2-butyne molecules appear as double-lobed protrusions due to two methyl groups, which provides a clue for determining the adsorption geometry of the molecule. 2-butyne binds on top of a Si dimer through di-sigma bonding between the C=C bond and the Si dimer. This is the only binding structure formed at room temperature. In contrast, acetylene is known to have several different binding geometries on Si(100)-(2x1). The exclusive formation of di-sigma bonded 2-butyne is explained by the calculated adsorption energy of 2.66 eV for the di-sigma species, 1.89 eV for the end-bridge species, and 0.67 eV for the r-bridge species. (C) 2003 American Institute of Physics.