The covalent attachment of diacetylene (HCdropC-CdropCH) and the formation of a C=C-CdropC structure on Si(1 0 0) have been investigated using high-resolution electron energy loss spectroscopy (HREELS) and density functional theory (DFT) calculations. The HREELS spectrum of the chemisorbed monolayer shows the coexistence of dropC(sp)-H and =C-sp2-H stretching modes coupled with the concurrent observation of the CdropC and C=C stretching modes. The experimental results unambiguously demonstrate a [2 + 2]-like cycloaddition mechanism for diacetylene chemisorption on Si(1 0 0) through the binding between one of the CdropC groups and a Si dimer, consistent with the prediction of DFT calculations. (C) 2004 Elsevier B.V. All rights reserved.